Well, not lynching your scumbuddy by mistake is pretty important. Otherwise, there's not much of an incentive.
This doesn't matter in mountainous though, because unlike setups with roles, there is no advantage to ending the game with multiple mafscum so you should never compromise your position to avoid the lynch of another player (and it's tremendously difficult to compromise your position to avoid the lynch of another player without actually compromising the position of both players anyhow).
The only reason scum ever need to 'play as a team' so to speak is when they are facing powers which (may) prevent them from winning the game with one player, since otherwise any game which ends with at least one scum alive is a scumwin and the best way to do that is to have very towny scum and the best way to do that is to have scum with very towny information models and playstyles.
There's an argument for not looking at it until night one. Mafiascum GMs used to make you respond to your role PM with your role name for this reason (also to confirm that people were still available to play).
Given that it is technically infeasible to avoid checking your role for the night, the question then becomes:
Is there ever any reason to use scumchat in mountainous mafia?
Every spec of information you have as scum makes your play less like your actual town play.
In writing this though, I've had insight into the issue and the insight is that persuasive position is more important than technical position re: information, and thus that an increased information base is very useful to scum seeking to influence other players.
To give an example which might highlight what I mean, there was a period in P25 where both Leafsnail's and my play greatly deteriorated compared to our previous efforts in the game (and his latter efforts) and during that period neither of us did a thing which was out of character for being town (since, indeed, we were town, and playing the way each of us would play town at that time) but both players made a number of mistakes which weakened the town and furthermore drew suspicion onto us. It is not in avoiding this that an informed scum player has the advantage, but in recognizing this in other players and so being able to influence those players more effectively by targeting their actual position without having to hedge their bets against multiple possible stances.
Also I suppose, one's relative confidence in the quality of their play over that of others would lead them to play differently for given roles. A player confident they could reduce their chances of taking a scumkill without giving out unacceptable reads might have reason (in their mind) to check that they are a town PR, just as a vanilla townie might wish to increase their nightkill chances (and be confident that they can out WIFOM scum).
I suppose the point I've come to is that scumhunting is not very important at all for town's ability to win the game
with optimal players since optimal players don't leak as scum, but
perfect players can simply persuade other players regardless and moot the whole issue - in other words, I'm looking at edge cases which don't exist and have been trying to step back and apply play insights from those stances onto the game as it occurs (imperfectly and suboptimally).