[apowell]

By suggesting several possibilities, you're saying "these times really do work for me, so if you pick one, we're set, you can put it on your calendar". When you say "let me know when works for you", you're still leaving open the possibility that you're not actually available at the time they choose, possibly leading to more scheduling emails.

Plus, when I'm on the receiving end of a "let me know when works for you", I feel a little presumptuous assuming that I really can pick any time and that person is going to clear their schedule for me.

[vacanti]

You also have to wait for the person to confirm before you can really put it down on your calendar definitively.