@taltimir
Firstly, there is no unit dispel in the game though it sorely needs something to serve that function. The current dispel only works on city enchantments, please check your facts before posting.
Secondly, you say these spells are useless against the AI although Brad hasn't finished programming it yet - he hasn't gone into much depth on the magic side for sure. No doubt at some point he will programme the AI to use the enchanted & equipped supertank strategy before FE hits release, even though it's a cheesy strategy.
Thirdly, all the spells are tactical for that battle only so you don't need to recast the enchantments after the battle and you don't keep buffs from mimic after that battle (perhaps that was unclear in the original post, sorry about that). You could stick casting times/ resistance checks/ cooldowns on these spells too but they are intended to be utility spells as counters against cheesy supertank strategies.
Fourthly, why the hell would you want to cast celerity 20 times anyway?! That's a ridiculous waste of mana that leads me to assume you are playing a heavily modded version of the beta
Fifthly, those ultrabuffed supertanks will still be awesome against any other tactical battle
Sixthly, they will still be good against a caster with these spells as they will no doubt be high level and have high level magic resistance and partial elemental immunity plus access to counterspells (the spells I suggested should prob all have casting times). They will still have access to amazing equipment that they've farmed from monster lairs too
But most importantly, it's about introducing a check to help balance the game. Derek has stated that he wants the game to be about building when it is currently an RPG with a town construction side-game. There are some players that do not train any units in the entire game and Tuidj stated that he won the game with no settlements at all - this is unacceptable that large elements of the game are simply marginalised. Part of the answer is to strengthen those elements of the game but it is also important to have direct counters to supertank strategies.
It's pretty harsh when an opponent casts growth and giant form on their dude and there's no counter to that. It's also harsh when your guy has been shrunk/gravesealed and there's no answer here too. The only reason you're OK about these things is because the AI is not using them yet; when they are using them properly you might realise there is a need for counters. Incidentally all the best strategy games include counters against other units and tactics.