If your allies don't sacrifice, the powers are pretty weak compared to other things you do. If they all sacrifice, it becomes very powerful but harms the party. So either the players refuse to sacrifice, and the warlock feels he's wasted a power, or the players feel used that they "have" to sacrifice "their" hitpoints.
And balance aside, its an ego thing. Many players hate when other players can affect their character. Even things like bluffing or intimidating a fellow group member can ruffle feathers. Now before people start screaming that their group is wonderful and perfect and everyone loves each other, the reality is that does happen in many groups, and there's no reason to encourage it.
Would you be willing to have your character provoke an opportunity attack to set up flanking for the rogue? If you can imagine doing that, it shouldn't be so onerous to power up the dark pact warlock now and then.
If those powers were kept in, I would use healing surges as the sacrifice instead. That way, players at least are not getting hurt in combat, it just means they have to rest more frequently.
That's a higher cost. The damage done to each ally by the powers we've seen so far is on average much lower than what they'd get back from a healing surge.