I've done trim carpentry on tons of new houses, some very high end, and they still used MDF which IMO is really stupid, as that stuff is just downright nasty.... Ok it does have a few apps.... but very few... but we never primed the ends, insides or nothing.... We always used coner blocks on the inside, but I've never heard of the BB shrinking that bad... Oh and none of these houses were "climate controled" so the material was the temp and humidity I'm sure swung quite a bit, but we never had any problems that bad. Cept if we cut twice and it was still too short lol.