Fortify Destruction in enchanting reduces cost of the spells. Cost reduction is probably most desirable thing to have as a mage in Skyrim. And it becomes very OP if you get it to 100% with enchanting. Without enchanting, best you can do is 75% and that happens around level 50.
Fortify Destruction in alchemy increases damage of the spells. There are weakness to fire/frost/shock poisons that boost damage of your spells also. Dualcast Incinerate does about 200 damage with perks, then it goes to 500 damage per shot with fortify potion, and the damage goes up to 1700 per shot with poisons.
No, it isn't desirable at all for most people. Just because some people like to cast for 0 doesn't mean all do. I don't abuse mechanics like that and I think it's stupid, it renders some perks useless and every point you've ever put into magicka useless. I for one do not use enchanting because making spells cost 0 is just game breaking.
Also, as a pure mage character I do not use Alchemy or Weapons to apply poisons so that is all out of the question. Destruction should scale and it lacks any sense of progress. Not to mention, the fact that Alchemy has weakness to frost/fire/shock and Destruction doesn't have spell equivalents of those is a mockery of magic. It's a complete joke. Alchemy has weaknesses and destruction doesn't? lol.
Your math is also completely off, you're not calculating it correctly and Master difficulty lowers your damage by half. Here's what Destruction looks like after:
1: Applying 74% weakness to poison.
2: Applying another 74% weakess to poison and 74% weakness to shock. (Boosted by previous poison.)
3: Applying 74% weakness to magic. (Boosted by previous poisons.)
4: Using a Fortify Destruction 185% potion.
Master difficulty vs an Ancient Dragon (3k health)
Video: http://youtu.be/qPCj9zrFXoE?t=2m49s
Sorry, but if you compare that to One-Handed/Two-Handed/Archery sneak attacks nearly one-shotting Ancient Dragons, the damage is extremely sad, even AFTER abusing all of those poisons, having to use a weapon as a MAGE, having to use alchemy as a MAGE, etc. something a pure mage might not want to do from an RP perspective.
Destruction magic lacks any real sense of progress in that it doesn't scale. All other offensive damage dealing skills scale as you level them up, except for Destruction. All other offensive damage dealing skills have a plethora of ways to boost their damage (especially without going to lengths of breaking RP) that Destruction does not. It's quite obvious Destruction is the weakest link.
Some might argue that Destruction is balanced just right, and the other skills are simply overpowered. That may be. However, Destruction still lacks a serious sense of progress which is a key element that every RPG should have, and for some reason they forgot to apply it to this skill. Don't even get me started about how gutted down and simplified the magic in Skyrim is in general.