After deploying your weapon on a surface you can no longer access your grenade launcher fire-mode, which makes for an awkward moment when you want to fire your grenades and have to first un-deploy your weapon and then switch fire-mode before firing.

It's really just a bit of an inconvenience... but if there isn't some reason that you can't access it other than an oversight...? (Perhaps due to the way a grenade launcher zeroes and must aim in the air, maybe it would bug out the deployment system?).