Dev and IXC only add about 0.1% more to your profits and it's another coin you have to send to an exchange and then dump for Bitcoin (assuming you only want BTC). I think the uptime and security affording by this pool and others like Bitminter and Elegius exceed the paltry add on of those coins.
Not counting uptime/security (unlikely to be affected), the real issue is extra load on the servers. As you pointed out, DVC+IXC are virtually worthless. I'm not even sure if they're 0.1% actually. Adding extra coins to each pool server means each server will be wasting time accessing the SSDs/RAM for those coins, a few CPU cycles, and some bandwidth every time they get a block. That will at least minimally impact bitcoind performance. How much? I can't say, and nobody else really can either. But it is a non-0 impact, and for how worthless they are, I'm not willing to risk ANY performance for junk coins on BTC Guild.