LED,
No, a bivy sack isn't overkill if you are hiking in an enviroment that warrent's it. I love my Mountain Hardware Conduit SL bivy sack. I have used my bivy in some pretty bad conditions with and without a sleeping bag. I don't carry it in the summer in AZ, but in the winter it adds 10+ degrees to my bag.

I am also a fan of the hammock, especially in thr summer... In the winter hammocks can be way too cold, even with a bivy sack.