I think incompetent but nice can also lead to morale problems. Not as bad as the jerk situation, but eventually everyone notices this one person who does no work but is still getting paid. Which can lead to jealously and feelings of unfairness.
It gets worse when they become the norm. Then the rockstars start to think “why am I doing the work for five people but getting paid only 20% more” then they leave.
"Doing no work" would be great. Understanding the commit they made last thing on Friday, which is incomprehensible and has broken your work, is not so great.
In my last job, I worked with three incompetents, none of whom were jerks, in a team of ten. I reckon the rest of us had to do twice the work to navigate their mistakes.
