Daily log in gifts entice players to log in everyday, player retention is something the investors want to see high.
Free heroes are not really for us. They are advertising. The sites with the keys get new members signing up, which means their site is larger, more people find out about MH and have an easier time getting into the game because of a larger starting cast.
Severside buffs cost them nothing and people may be more likely to try the game when they see these buffs active.
I'm not talking about physical cost to Gaz. Obviously that's nothing when it's just server-side toggles and generating virtual keys.
I'm just talking from a game design perspective. I'm honestly curious what are you suggesting? That they do (for example)... the crazy Big-10 buffs literally every single day forever? Give away free heroes every single week?
I mean to me that would be crazy. Where does it stop?
Let me ask you something else... how come no other game does this? Lots of people can be armchair game designers, but when the vast majority of MMOs and online games don't do examples like above, shouldn't that tell you something?
Maybe I'm jumping to assumptions about your ideas, but what exactly do you want them to do? The Big-10 buffs were ridiculously over the top, I sure hope you don't expect that every day forever.