I am a tank Powertech (Shield Tech) and have opted for the classical 31 / 8 / 2 set up (skill points).
And it seems no problem at all soloing the PvE quests (I only die when I do not check what I am attacking first: for example if I do not see that there are healers among the Elites or whatever, and I do not take them down first). I am sure I go slower than pure DPS, but I also am pretty sure I can deal with stronger enemies than pure DPS.
So, pure tank is no problem at all (we get Mako the Healer as our first companion, and I have never once used any other).
re. DPS as Powertech: I have no personal experience with that, but I know there are builds, for example the Shield/Pyro Carolina Parakeet build (21 / ? / ? I think).
My feeling is that those builds will not be able to tank in FlashPoints etc., and I like the fact I can (maybe a bit slow) solo everything, and then join as tank in FPs.
In PvP, I imagine the DPS Powertechs do more damage than I do... but, again, I tank in PvP, and it works very well: I am not that great in taking an enemy down, but when I shield a Healer (or DPSer) and reduce the enemy's damage, it helps a LOT.
The most important, IMO, is for your to feel what part you want to play: a tank (go for pure ShieldTech tree, works great in solo PvE, group PvE, and PvP if you fight using your skills ie. Guarding, Taunting...) ; or a heavy armor DPS (I imagine going Pyro' ?) which might work in PvP (as I said, never tried that side so cannot say).
But, no, you do not need Retractable Blade as a Powertech.
You do not need it at all if you go tank / ShieldTech.
And, maybe you can "need" it if you want to do a more DPS less tank build?