How you fill a bag depends on the design of the bag.
Some are designed with open bottoms -- these are easy to fill from a reg, but they have to be kept upright on the surface, or they spill their gas and sink.
Some have a duck-bill valve in the bottom. These are filled from a purged reg, but they will hold gas if they fall over. They can't be filled any other way, though, and sometimes (as in very cold water) you don't want to purge a reg for very long.
Some only have an oral inflate tube, and others (like my favorite, the Halcyon bag) have a connector that permits oral inflation OR attachment of an inflator hose.
The advantage of spools over reels is that a spool can't jam -- as long as you keep your fingers out of the hole in the middle, the worst thing that can happen is that you will drop the spool. And in that case, the bag goes up and the spool comes back down, so as long as there isn't a lot of current, you may even be able to recover it. If you are not shooting bags from major depths, a spool is absolutely great for the task.