Yes you can currently use the non-steam client; I haven't heard anything about plans to fully integrate the game into steam, so for a while now hopefully.
I would imagine that they would not remove the option of having a stand-alone client. Steam makes it fairly easy and simple to update from my understanding, but the main stand alone is the main stand alone like most other games that are hosted on steam that are multiplayer online.
I do hope that's not going to be the case, for some reason playing with Steam it told that "Memory is full", but with stand-alone client there's no problem. Problem probably lies in my really awful laptop.
I saw a few of those, but I'm not entirely sure when or why it is happening. It is possible it is due to actually having full memory, but I don't have enough information about it at this time.
Yeah it's hard for me to log in with steam since I'm on a Mac. So pretty much the stand alone will be here for awhile (if not I'd be very sad panda).