It’s more than just comments - usually stuff like this is organized in the form of a pinned poll with a comments thread below. As people add comments they also change their poll selection. So you end up also having a real-time view of what people are bringing as well as an audit trail.

You could have the same thing in an email thread if you linked a spreadsheet in an email, but people are less likely to keep both in sync because the things are in two different contexts. And even so, what would you use? Google Apps? That’s just trading one data-hungry overlord for another.