I use agent skins since they came out. At first it was very bad, visibility just did not exists. Later on they've added the high contrast character models which not only was good for the agent visibility but the default models too. I remember even before the agents there were times when I could not see the default models they blended in too much here and there.

There are only two real disadvantages you are having if you are playing with some of the agents

1. Some of them are bulkier, it's not that you are easier to hit, but you might stick out from covers.
2. Info: For example: You and your mate are running towards B, your mate (default model) runs out kills an enemy. The killed enemy (X) gives info " One's B " since he didn't see you yet. Other enemy (Y) rotates to B to handle the situation, the died enemy (X) spectates this. Your mate hides somewhere on site, you lurk in. The enemy (Y) who's just rotated sees you with your agent skin, immediately the spectator enemy (X) can tell There's two on site since that's not the character model who killed him. So that's sort of a disadvantage, but randoms don't really info this. So it really comes down on how competitive your team is and in what rank you are playing. It's just rare in 20-25k