The tau are pretty insignificant on a galactic scale. They have a laughable population density and 18 major worlds. Imperium has a billion. Their population is is a ripple on the ocean that is humanity. The only numbers I could find suggested that the Tau are less than a billion in population. GW never was good at figures.
The Tau survive because their main threat, the Imperium, is also dealing with countless other threats. When or if the Tau became a bigger threat, the Imperium would subsequently rally forces and deal with them. At the current stage they aren't a major race for any reason other than to sell models.
The Tau's caste system and their methods of keeping control are highly shady too. The ethereals have some sort of mind control over all other castes that causes them to go bathshit and emo when they die. Its not direct mind control, but they have a heavy influence over the tau's general mood. Plus the whole greater good thing has been shown to be little more then a way to buffer the masses with propaganda, and thus make the tau seem attractive to detractors from other galactic civilizations, which is actualy pretty clever considering how small they are, so absorbing people from other factions is the optimal strategy.
The Tau Empire's way of manipulating everyone was exposed by one of the least shitty of the Tau: Commander Farsight, which still leads his farsight enclaves in the minor territory he retook from the imperium and the orks.
Still, the Tau aren't even close to being a real threat to the imperium, not while stuff like tyranids and necrons are around, IE galaxy spanning horrors that can possibly destroy all life if fully manifest.
...None of that is true. Well, the Imperium being able to wipe the floor with them if they got their shit together is true. As is the bit about them being Orwellian. I do find it funny how everyone seems to like Farsight and hate the main Tau Empire. I think it has to do with them being naive and small and yet still a faction in the otherwise grimdark and over-the-top 40k setting, since they're there at least in part to serve as a contrast.
The Tau are mostly insignificant compared to any other faction save the Dark Eldar on any major scale. They have a far less sardine-canned population density(I can only presume that's what you meant by 'laughable population density', since the idea of Tau having less population than currently on Earth when so far advanced, and on 18 planets, seems like hyperbole to me) and more than twenty fully developed Septs, plus other alien allies. A single Sept will usually consist of one or more star systems with appropriate mining facilities, colonies, fully settled worlds, and the like. Compared to any similar alien empire that is not a galactic threat, they are the largest and most powerful. They are also growing at an astonishing rate, thanks to the Imperium's distraction due to the everything, at the moment. And in the Damocles Crusade, the first major Sept the Imperium reached, the Tau started slaughtering Imperial forces. They made gains, sure, but even if they didn't have to leave to deal with a Hive Fleet, it's still unlikely they would have been able to keep up the momentum to get hold of a second major Sept world. Something will always eventually come up that requires the Imperium's attention and diverts it; that's the reason they can't focus on any one specific faction and kill them all. Imperium has one million worlds or so to look after, after all. Population compared to the Imperium, if you count only actual Tau? Probably around half a percent. If you count Xenos/Gue'La allies? Probably around 1-2%, depending on the numbers of Xenos. Plus, all the worlds they've been gobbling up in the Third Sphere Expansion, which has been promising to be the largest and most exponential one yet. They're also a shitton more efficient than the Imperium, are actually advancing in technology, and might end up being able to get AI that doesn't rise up against them, simply by treating it as an equal. Tau would be First Among Equals, but still.
You may dislike a faction, and that's fine, whatever. I don't particularly like the Dark Eldar, but I still can deal with them and understand why they're part of the franchise. To dismiss them as utterly irrelevant and incompetent or the like is rather rude.
And while I will fully acknowledge that the Tau are not nice guys by our standards; they are far, far nicer compared to the Imperium. Ethereals have influence over the other Castes, and their death is a terrible loss, as their guidance and knowledge is lost; in addition to their being somewhat similar to Living Saints in terms of importance to the Tau. I'm not certain where you're getting the idea that the Greater Good is a giant lie, as I don't see how that's the case. Is it also propaganda? Certainly, but when the benefits include not being worked to death in slave pits if you join our Empire, you don't need all that much propaganda. They'll quite often (believe) they've assimilated factions wholesale, such as the Kroot and Demiurg, and while this is sorta technically true in that agreements were made such that they'd all be citizens of the Empire and the like, it's not as though Kroot aren't found elsewhere in the galaxy. Plus, they just don't have anywhere near as much of the rampant xenophobia/genocidal tendencies of basically every other faction.
As for souls, they have souls, they just don't have much of a warp presence. Daemons will eat Tau souls just fine, it's just that they're hard to notice, and they aren't much of a meal anyway, as Gig said. My personal theory for the Ethereals is that it's either pheromones that also affect themselves, or they're actually a Tzeentchian plot, and he hid their Warp presence, basically, so that his plans couldn't be interfered with in The Great Game. Tau are the one race kinda based around hope, change, and progress, after all.
Oh, and did I mention that from what I know, on the forums I've been on, I have a fairly cynical view of the Tau compared to other people who play(or would play, if I had enough money that I could use it as kindling)?
Tau are weak on the BANEBLADEHUEG galactic scale of 40k, sure. But on the scale of the battles fought in the tabletop? And on a long-term scale, if the Imperium doesn't rally to deal with them before they gain the ability to, say, send stars into supernova on purpose(yes, this is something they're working on)? They count as a threat. Not a giant one. But one similar to the Eldar or Dark Eldar; they make it harder to deal with the really big threats. Of course, just like all the other factions keep the Imperium from wiping them out, the Imperium also serves as a buffer and prevents the other factions from wiping the Tau out. Tau have experience fighting Orks, but WAAAGH!s are dangerous no matter what, and Tomb Worlds can still wake up. Tyranids are a duh.