Task stealers! :-D
Allowing others to edit task description allows them to add details (yay!), but also to reframe/change it to a different purpose.
For T229 started as bug for missing CodeReview-1/+1 in Phabricator.
In T229#21 details have been added to the task description. That's great. Wohoo!
But in T229#36 the task was turned into a huge, different task beast.
If I want to bring the discussion back to CodeReview-1/+1, I can no longer say “Let's come back to the task's description” ... instead I can edit-war the description :-), or I can create the original tiny task about CodeReview-1/+1 once again (as separate task).
I prefer the Bugzilla way. Someone describes a problem, and afterwards details/corrections can only be added as comments. That's sufficiently simple. And things do not mutate over time.