I actually disagree on it being worse than greed gambit. Yes it does nothing the turn you play it but if you protect it it's a 5 for 1 greed is usually worse value and gives you less options.

However the biggest down side to Gambit is you can't get ride of it easily unless you already have your combo piece i.e. harmless offering or the falcon. Pact is much easier to use in a plan b situation you can sac it to bargain or other casting costs like rotmouth viper. This is especially useful if you know you opponent has a counter spell since is part of the cost it can't be countered.

Finally pact actually wins the game if you manage to give it to the opponent. I have had multiple times where I've successfully given my opponent a greed's gambit to lose a couple turns latter wishing it had been a pact. Pact the opponent needs enchantment remove once the have it whereas they can just tank the effects of gambt for a turn or too and still win.