I feel like I'm in the minority here, I love me some Warlock, but the bag of hex chickens thing really rubs my rhubarb the wrong way.
For me, I just don't see the problem.
1. It's kind of thematic - sacrificing a chicken (or whatever) to fuel a power is just the right kind of icky;
2. You still have to expend resources: a 1 hour rest is not nothing and can cost valuable time. If time isn't an issue then then 1 extra slot probably isn't either;
3. The concentration requirement makes it a serious trade-off. While it's up, no spider-climb, invisibility, gaseous form etc. And unlike hunter's mark - out of combat utility is limited to non-existent;
4. Outside the trade-off. Concentration requirement is no joke. It's basically become a running joke in my group how often the ranger loses hunter's mark.