Probably smart for him to do. When a guy has as much good looking tape as he does, 40 times and broad jumps really only serve to nit-pick a player and bring his value down.
That is not true...for two reasons. McClain's tapes aren't anything special, and secondly the combine is not designed to nit-pick players but for players to showcase their talents. Scouts want to see a guy run the 40 to compare his talent to a lesser talent. Is he running a 4.75? If so, how does that compare to say...a Micah Johnson? The only way a combine workout would ever hurt a player is if he doesn't attend, or gets injured while doing it....but the odds of hurting yourself severely are less likely than at practice because there is not contact.