I have ridden thousands of miles over 10 years on similar "crap" derailleurs (actually I think my FD is an even lower level C-101 or something), and other than needing to adjust the front derailleur once, they have shifted wonderfully. I have removed the RD a couple of times to clean it and lube the bushings while I was replacing the cables, and cleaned/lubed the FD in place at the same time. I bought a new RD, because I was thinking of replacing it, but decided it wasn't worth the time to fix something that wasn't broken... so now I have a spare in case it ever does fail (there are no signs that failure will happen any time soon).

I have seen 20-30 year old Tourney RDs that still shift fine when covered with rust... and Tourney puts the low in low end.

I strongly suspect that a significant portion of those who say anything below Deore is crap are just repeating what they heard from someone several steps removed from a sales guy who was trying to convince a cyclist to buy a higher level bike... Really, how many people do you think have done an objective comparison of the different levels of Shinmano components??? Maybe someone did, but the chances are more likely that Shimano (or the local bike shop) is good at marketing their higher cost components.

I think the better way to look at it, is that Deore is a sweet spot in the line from a cost/benefit point of view (as 105 is viewed in the road groups)... so, if you need a group, check them out. It definitely should not be interpreted as meaning that anything below that level should be replaced... unless you're a sponsored rider in which case, you should definitely insist on the top of the line!