Reggie is only 6' 8" and 245. He works very hard on both ends of the floor, and is physically aggressive - a real disruptor of another teams offensive continuity. He is just a bit lighter than Elton Brand, who is the same height.
He is probably the best player on the Sixers to guard a LeBron James, or some of the other larger three's. At the present, Thad Young would be overwhelmed physically by a LeBron, and I doubt Speights would do much better. Since loosing Reggie to foul trouble doesn't hurt your team on offense, you can afford to have him play as he does now. VC is listed as a 2, not a 3, that spot was RJ's.
Reggie is too small to be a Center, and although he holds his own at the 4, he may be quite well-suited to being a defensive 3 who can supplement the rebounding and interior defense. It is reasonable to question his quickness or his ability to stay with some of these players, but we're talking about a guy who is now the weak link on the team, but all of a sudden the team is thin at the 3 spot since we lost Carney and released Giricek.
It isn't clear that Thad is ready to be a 30+ minute player. Whatever minutes he can't use have to go somewhere, and unless the team wants Green at the 2 and Iggy at the 3, those minutes are going to go to Reggie. With the additional scoring strength we have inside with the addition of Brand, a line-up of Smith, Brand, Evans, Iggy and Williams would have plenty of scoring ability and create match-up problems for other teams. That line-up is quite a physical group as well, very tough for other team's second-string players. If you instead, go with Smith, Brand, Iggy, Green and Williams you have a less physically imposing group and are somewhat undersized at the 3.
I suppose against certain teams, putting Iggy at the 3 may make sense, and that would give Cheeks the opportunity to play Green more. But when you look at the elite teams, they often have larger players at the 3. Who do you want trying to stop 6'7" 235 lb. Paul Pierce, Iggy at 6'6" 207 or Reggie Evans at 6'8" 245? Would you rather have Green trying to stay with Ray Allen, instead of letting Iggy take him inside? Not to mention getting physical with Pierce which is probably the only way to slow him down!
He isn't too big to play the three, the only question is, is he too slow. He probably will be against some players, but since he's second-string, chances are he's not going to be lined up against the elite 3's too often.