there is no general reason why aliens shouldn't be allowed to evolve in any direction.
in the unidirectional case, being stuck with a subset of alien classes creates annoying situations for aliens in matches with very low player counts, particularly 1v1s. (however, being stuck with a construction kit for a prolonged amount of time, creates similar annoying situations for humans.) so allowing bidirectional evolution would eliminate this issue, but beware of the then-needed rebalancing changes — but not that balance isn't broken in the first place.
however, unidirectional evolution sounds more interesting, and would be more interesting, if all of the Tremulous gamelogic were unfucked.