[jimbob]

I personally prefer this approach over separate racial trees because then when you capture an enemy vessel (i.e. the borg capture the enterprise or vis-versa) you could disassemble the ship to reverse engineer the technology/components. As long as the players are honest and don't cheat by researching that branch further, you can get some fun assimilated/hybrid ships floating about the universe.
But with strictly exclusive tech trees, the Borg (or any other race) can only hope to assimilate the most mundane (common) components, like engines, shields... the boring stuff that everyone researches in common.