It really depends on the band, the tour and how extensive it is, when it comes down to a deciding factor. When the SFU/Dying Fetus tour came through DF didn't have shirts with the dates on the back but SFU did, didn't buy a shirt from SFU but did for DF. It would be nice to see all bands have at least 1 piece of merch with dates on the back but it be in very limited quantities. Hell I'd even like to see shirts for specific dates kind of like what iron maiden does.
My favorite piece of tour merch is ties between the Job For A Cowboy winter 2010 tour which is the design of this poster
and the fall 2009 Trivium tour shirt which is pretty cool, which I can't find a picture of.