Just to make my position on this clear, as I see Dwarf Fortress as a toy, the idea of cheating at it is a little... odd.
Now, if the game you want to play is "keep making fortresses until you eventually see everything", I can see the appeal, and for that game using regional prospector would indeed be cheating. But since the only person enforcing the rules is you, it's not my place to decide how you should play.
If you want to play a different game with DF, such as "mine adamantine and avoid destruction by demons, while using as many map features as possible", then there's no rule against regional prospector.
Now, if this were a competition, there would be completely different rules...
As is, though, I occasionally use Regional Prospector if I want a fortress with HFS, but otherwise don't bother with it.
Well, cheating in Dwarf Fortress is exceptionally easy. I view the regional prospector as "the dwarven Geologist"
Certain things happen in reality that make finding certain features, paticularly magma features, much easier. Finding a volcano on earth is as simple as finding a hot sulpher spring. Underground lakes and rivers I could understand not finding, but geologically active areas, where magma would be found, have certain things that happen around them, even if there is no volcano.
Realistically, Dwarves setting out would probably prospect a large area before siting mines. And they would certainly look for the resources required for a city before finding them. Since Dwarf Fortress lacks a "Prospector Mode" with Adventurers, such activities as using regional prospector fill the hole.
When my adventurer in adventure mode can smell sulfer vents around volcanos and watch for sections of waters that don't freeze when they should, then we can discuss how regional prospector is a cheat that doesn't make sense. You want to discuss cheating, how about fortresses that have inpassable entrences? The ones with draw bridges across chasms, and those with walls instead of doors. That's more significant then making up for the fact that an Adventurer cannot prospect or the fact that most computers cannot handle 17x17 size sites.
I'd happily make massive Engineering projects to bring magma to my fortress rather then use regional prospector, but my poor computer couldn't take it!
So stare down your nose at people all you want. I've made successful fortresses on Terrifying coasts without a drop of fresh water, Using regional prospector to give me flowing magma and water to allow a total sandbox area isn't something I will give up, until I can go from adventurer prospector to dwarf fortress that is...