Since all HAVs have the same jetpack, with unlimited fuel, the only reason to upgrade is for the HP boost you get with better models. So upgrade it when you get the money.
It's only use really, is opening boxes. Maybe killing other humans, but it will take FOREVER to kill a Panzer with one of these. Not very important to upgrade.
Your main means of attack as a human. These can actually do decent damage to a Panzer... about as much as the GRL weapon. You have unlimited ammo, but be careful - the explosion from the grenade or shrapnel from other Panzers can kill you very quickly!