You can, and in situations where the computation is unavoidably long that's what you'd do. But if you can do a bit of work to guarantee the computation is fast then it removes a potential failure mode from the system - a particularly nasty one at that.
If you forget to dump the key (or if the deletion is not clean) then you've got an absolute whopper of a privacy breach.
Also worth noting that you can't dump the key until the computation is complete, so you'd need to persist the key in some way which opens up another failure surface. Again, if it can't be avoided that's one thing, but if it can you'd rather not have the key persist at all.
If you forget to dump the key (or if the deletion is not clean) then you've got an absolute whopper of a privacy breach.
Also worth noting that you can't dump the key until the computation is complete, so you'd need to persist the key in some way which opens up another failure surface. Again, if it can't be avoided that's one thing, but if it can you'd rather not have the key persist at all.