Wakeups are a guessing game. The defending player is forced to try and guess what the offensive player is going to do. If they think they're going to attack immediately, throwing out an invincible reversal is great. But if the other player does nothing, they then get punished for it.I'm a Vega player, I don't know what you mean with this "guess if you're going to attack or block on his wakeup". Never been in that situation before.
It's risk/reward. The issue is that being able to cancel out of the reversal lets you get the best of both worlds. If they attack? Reversal! If they block? Cancel->Backdash. The penalty for being wrong is gone, and it's made even worse by the ability for many characters to Ultra their opponent after a successfully cancelled reversal in SF4, so it becomes two potential outcomes of "EAT MASSIVE DAMAGE" or "SAFE", which is completely off of what the risk/reward should be for that type of attack (Momentum change + small chunk of Damage vs going to eat a lot of damage)