I talk a lot about how much I like tree stumps, so I'll talk a little bit as to why it's not necessarily a "harmful" civilian role. Granted, the obvious drawback is that a stumps activation removes civilians of one precious vote (and sometimes stumps don't count as living civilians, affecting mafia win-cons), but it provides a couple of unique benefits in return.
* It gives the stump confirmed townie status which, if the stump is under pressure before activation, can help the thread get back on track.
* The immune status will remove any "restrictions" the player has to worry about. For example, some civilians withhold everything they're thinking because they don't want to draw attention as a night kill target. Stumps can word vomit all over the place and not have to worry about the repercussions.
* Much like a thread speaker role, activated stumps become a trustworthy player for civilians to bounce ideas off of or seek inspiration from. Huge benefit in this in getting townies engaged, out of tunnels, and whatever else can be detrimental to the civilian effort.
It's a bit awkward in deciding whether the benefits outweigh the costs with that kind of role. In the end it comes down to exactly who has that role and what the conditions for activation there are. On the former, I think some players would do the stump role justice and others not so much. A JaggedJimmyJay would make the most of the role, whereas a Dom might not. It's reliant on the player activating and being able to produce enough valuable content and keep players thinking and engaged to account for the vote that they've lost in activation.
I won't argue that stumps are a highly rated civilian role, but for me they fall on barely civilian, transitioning depending on how well it's used.