My loremaster experience is rather limited but most other classes already needed two sets of LIs/gear anyway now to be effective in different roles (even if switched to on the fly without retraiting, i.e. my warden carries a couple of armour pieces and jewellery to switch out and of course a tanking weapon when switching from landscape stuff to tanking, same for the runekeeper and minstrel).
So putting that on the new trait system is not fair.
You will have less choice for a certain setup however you will win the chance to trait specifically for different encounters in a single instance without leaving it. I am not sure if there are any encounters that wold really profit a lot from that but we will see.
How effective we will be by switching a role on the fly during an encounter we will see but I expect it to be much less effective than currently because we will miss some skills (i.e. talking about dps runekeepers switching midfight to help healing e.g.) on top of not being much effective already in the current system because of the traiting.