agree with you there, with something like marinated chix breast even though my food saver has a moist dry setting it will suck a little bit of marinade up into a drip tray in the machine and you will need to clean/wash out the trayI only have the vacuum bags that come on a roll, so yes, I have to size and seal before I can do anything with them. I suppose if you had pre-made vacuum bags, there wouldn't be any difference in time or convenience.
The big leg up that the immersion seal has over the vacuum seal is that you can safely seal liquids in the bag without having to resort to freezing them or buying an expensive chamber vacuum.