The M8 and M8.2 have a IR-blocking cover glass 0.5mm thick, which lets significant amounts of IR on through. The M9's cover glass is 0.8mm thick, and while it blocks more IR than the M8, it still lets a good bit of IR on through. Not *quite* as necessary to use the filter, but... I would like to block more IR in both cameras, so I leave the UV/IR Cut filters on the lenses whichever camera they're on... to also include the M240 whose cover glass is 0.7mm according to some info sources.
The stray IR has an effect on some colors, depending on the IR content of the light. Heat lamps, incandescent lighting, direct sun... Not just black artificial fabrics turning purple, but foliage taking on a yellowy-brown tint, pinkish skin tones, and generally slightly skewed colors of various sorts.
I thought at first I could get away with not using the filters... wrong! I set off with my new M8 to Malta and Sicily on vacation in 2008 and quickly found that what the experts said about IR was right after all. Similar with the M9 but not to the same degree.