It would require knowledge of your opponent and their tendencies to properly counteract their abilities in such a case. It'd be far more dangerous to try and counter an unknown in this case, giving increased credibility to something I saw elsewhere - questions about spies/scouts, and therefore intelligence.
Well, if you have full information about your opponent then the result would be exactly what you don't want - spending essence (although I imagine it would require mana, not essence) to counter a spell. And if you don't know, then it puts the defender at a huge disadvantage because you're relegated to either your opponent's spell go through unchallenged, or randomly guess and risk screwing yourself even more. I'd prefer a straight-up "spend mana to counter" system, although I'd prefer even more a system where different counterspells would function better against certain types of spells (some might specialize in dispelling enchantments, some against blocking or dissipating direct damage, etc). You could also throw in a mechanic whereby the type of mana you use to power your counterspell affects the outcome, but in a way totally transparent to the player. For example, you might be more effective at blocking your opponent's volcano spell if you predominantly power your counterspell with earth mana; water and fire could be a bit less effective and air even less. But, to keep things from being too much in favor of the offender, mixing water and air might be just as effective as using pure earth magic, etc.
