It's important to differentiate between The Back Room and the Smash Back Room here. The Back Room is for general discussion. The Smash Back Room is for Smash Discussion.
I am at this writing moment a member of the Back Room. I used to be a member of the Smash Back Room but I lost access randomly one day. I'm not 100% sure but I'm pretty sure it was due to inactivity because I hadn't posted in quite a while IIRC (so it wasn't like I posted stupidly and they went "Let's take it away!"). I've since not reapplied 'til just recently (like, two weeks ago or something) because I felt I wanted to see what's going on in the SBR nowadays.
So, in short, I'm a member of the BR, not the SBR (anymore). Also, anyone who donates or has someone donate money in their name and becomes a Premium Member automatically has BR and SBR access, IIRC. Said access is, of course, revoked once one's Premium Membership expires.
The SBR is for intelligent discussion without all the spam that the General boards have. Also, it's to weed out unintelligent posters, I guess. Likewise, the SBR is for intelligent Smash discussion. There are never any threads like "Smell the characters" in there. The SBR is where they discuss important things like serious debate about stage bans, banning of stalls and techniques, etc.
This, of course, creates limited usage. Few threads are ever active concurrently and there are also generally many less posts in there (like, say, a thread about stage bans in General Brawl vs. SBR, the General Brawl thread will outnumber the SBR-thread in posts 10 to 1 or something. On the other hand, all discussion is serious and for the most part intelligent ("I want to ban Skyworld!" vs. "I want to ban Skyworld because I hate it!" vs. "I want to ban Skyworld because if you break the platforms, it's an auto-loss for Olimar and pretty bad for Zamus and Ivysaur.").