I'm not a huge fan of most walkers and vechs to be honest; the idea of vechs is to have armor, thus making them hard to kill, but if you get unlucky they go down in a single shot, wheras if you have 20 Boyz, no matter how unlucky you get, they must suffer 20 wounds to be destroyed, that means they have to be shot at least 20 times (or 20 of them need to be caught in an AoE, but AoE's are another thing entirely).
High cost units tend to end up with massive targets painted on their faces, and most races have some long range way to deal with those targets. In 1000 point armies i find that you're usually much better off investing in larger numbers of smaller units.
I suspect the reason for this is because vehicles are horribly overpriced, this is because if you don't have an antivech unit, then they're basically indestructable, but if you do have one, then they drop like flies.
The Toaster Monolith is the only vech that i personally feel is reasonably priced, and that's because it's sufficiently Hueg that even if your enemy has antivech measures, it still takes a concerted effort to bring down, which, for 235 non phase-out points a pop, it bloody well should.
Basically, with the Deff Dread, is the Deff Dread as good as 20 Shoota Boyz without Big Shootas? Can it take as much punishment? Not really; if your enemy has no anti-vehicle measures then it's indestructable, but if he does then it'll drop right quick. Can it do as much damage? Nope; the Shoota Boyz with their 40 attacks will far outpace it, and the Shoota Boyz have the advantage of gradually reducing firepower, wheras once the Dread is out, it's out.
I'm not saying expensive units don't have their places, sometimes they're well worth it, and in larger games where you start to max out your troop allotment they become incredible, but i usually wouldn't consider deploying any walkers or vechs at anything less than 1500 points.
The other thing you have to remember in 1000 point armies is that with only 1000 points, a good hard vech is horrifically lethal, IF and only if it cannot be countered. As such everyone always ensures they have some hard-hitting anti-vehicle units in their 1k point armies, because you just flat out cannot afford to come up against an army with vehicles and not have any anti-vehicle units. What this ironically means is that in 1k point tournies, vehicles aren't very useful.
I should also note that some vechs, like Trukks, are the exception to the rule, as they're cheap enough and useful enough to be viable. IG's vechs are also cheap enough to be viable (and rightly so).
The other thing about Walkers is they still only move that puny 6' per turn, blah!