There are usually a unit or two on most missions that can bypass shields. All melee attacks (Juggernaut, Krogan), rockets (mercs, Geth), snipers (may only be percent bypass), acid (Rachni, Creepers).
The other issue is that you can't simply compare shields vs damage protection as all armor have both.
So then the next issue is "Does Predator L/M/H damage protection and shields outweigh Colossus armor damage protection + shields?"
Really I don't think it typically does for most of the squadmates, even the ones with Electronics and Barrier. At least not by the time you are able to unlock the good armors. It is probably close though, so on someone like Garrus or Kaidan it might work out depending on levels.
For Tali I think you are stuck with Colossus because of the limited selection of Quarian armor.
Ashley and Wrex are nearly always better with as much damage protection as you can get. They have better health pools, and with Immunity stacking they can be nearly invulnerable. This is also why many times I run an Ablative (or whatever the name is) in one of their armor slots in conjunction with a Med Exo (since they both also get health regen from a talent).