If you are doing weddings or group portraits, yes trade the 70-200 for the 16-35. The f2.8 versus f4.0 of the 24-70 makes a small but significant difference in low light of indoors, also you will find you occaisonally need the wider angle of the 16-35 versus the 24-70 or 28-70. The 24-70 will make it most times, but if you can't back up far enough, having the 16-35 can make the difference.
I have had occaisons where I had to swap out the 28-135 I was using (now using 28-70 in similiar circumstances) for the 16-35, because I would otherwise just not make it. However In a medium to large hall I shot my entire Mom's side of the family at a reunion (about 50 people) with my 28-135 at 28mm, so it can be done. [trivia: this is the first shoot where I noticed my 28-135 having a problem]. In the case of the 28-135, I had to use a pair of 550EX flashes to provide enough light.
So if I were shooting weddings I would get the 16-35 and the 24-70 (skip the 28-70 unless significantly cheaper in this case - I like mine, but the 24-70 would be better). Also I recommend getting the 50/1.4 or 85/1.2L (depending on your taste) for use in portraits and really low light situations.