It's just a stereo-typical thought of what I call an "older generation." In the past, once a scanlator started to scanlate some series, others didn't touch it regardless of how slow it was going. For me personally, if I wanted to scanlate some series but someone else is already doing it, I'd rather help them out than making it on my own. I've done that for many series, and when I had time, I even helped out with series that I wasn't so interested in. In my brain, I'm probably thinking if we have extra hands to scanlate one series in two ways, I'd rather use that resource to scanlate some other titles that are not known in English world.
Please note that I'm not trying to start an ugly fight or anything. I'd rather make good scanlations ourselves, so that other groups will just give up on their own. I think it worked well on other KEFI projects, seeing how Fairy Tail, Mx0 (even though we quit), and To-LOVE-Ru all had competition when it first started but they all gave up, let aside other projects from my previous group. That's why I joined Null secretly when Matt started his scanlation group. If it were not for him, I probably stayed away from One Piece scanlation D: But yeah, after all, Null has been scanlating One Piece since volume 20 consistantly, and I wanted to acknowledge their effort and help them out.
I am, however, aware of the fact that Rush's scanlation style difered from Null's, for them editing out SFXs and replacing it with English SFXs. Though, I've never seen any series being edited in two ways (edit sfx or not) aside from translation, and I understand people prefer one over the other.
Just a side note, probably many of the core Rush staff knew that I'm involved in Null, or at least I know Shinwei and Ivotas knew it before they started Rush (or at earlier stage). I even talked to them both, half jokingly, to stop Rush and join Null even if I knew both of them had problem with Null before :P
