mmmmm.... not sure. I think matt is right about post type really benefiting from timestamp. It's really annoying reading things that are old when you want new things and a person just came into gossip range of you. or rather, it's strange, and sometimes embarassing when you don't realise how out of time your reply might be.
It's annoying when a whole persons feed sits on top of all your other friends content, that makes me tend towards blocking to remove the noise.

I think if you remove timestamp you might have to make branch compulsory.
I think branch is actually way more interesting that timestamp.
Downsides would be losing ability to do any easy queries.

Of yeah I tihnk as the database gets older being able to say "only message published in the last year" might become really useful. I think this might be the case with all message types. Something like tags don't really need a timestamp to operate, but at the same time, if I want to see new tags my friends have been gathering how do I do that.

I think we need to imagine a range of scenarios and propose how to build those quiries. Or to spend a week re-writing modules and avoiding using timestamps to see how hard it is.
I am against anything that makes querying any harder than it currently is - either I don't have good strategies or some queries are really hard already.