Streamers can give their team a huge blow if they show everyone on stream which position a certain player is playing and who the certain player is.
For example, lets say someone is ghosting a streamer and see's a challenger player who is one trick pony for top lane, then they can use that and ban the champ that the person mains, during pick ban phase.
Also, streamers who stream jungle i would imagine are also a huge blow to theri team.
There are a lot of streamers, and it is improbable that anyone would just happen to be watching a particular stream of someone that they happened to get matched against (which also implies they're challenger themselves). Even then, they wouldn't necessarily know that it was them on the opposing team until they got into game.
Top tier challengers may get a feel for what hours their fellow challengers play anyway, and maybe perhaps try to do some target bans if they know it's relatively quiet hours and can guess just based on people with comparable elo who would be even on. Target bans are a thing in actual pro play anyway, so it's probably not a big deal to get used to them, and even the "one trick" challengers usually have a back up or two. Not to mention at that elo, people will almost certainly be able to play the OPs of the moment to their fullest potential, so by not banning <insert op champs here> in favour of banning someones one trick, you might still be setting yourself back when a different player carries with that OP.
Watching a stream and trying to play the game (at a challenger level no less) at the same time seems impractical. They might be able to cheat a little bit, but they are challenger, so what's the point of cheating? Challengers are really really good at the game, so they shouldn't need or want the crutches. Heck, many of them would probably want opponents to be playing their main champs just to test their mettle properly.
