Depends on the person and the circumstances of leaving. In most cases of firing or redundancy (vs. because they quit) it's safer to just pay them for the notice period but not let them to come into work anymore (severance). If they weren't that useful to begin with, you're not exactly missing out. If you're letting them go in some amicable way, it may be different, but I'd imagine you'd want to be very careful they won't do any damage.

It's safer if you terminate them immediately and just give them 30 days severance pay, which is the same thing.

I'm not sure I'd trust some people not to have a negative reaction and try to sabotage their project.

Agreed. It's easy to get worked up about paying for nothing, but the potential downside is often greater.