I think the key here is you may have a slight misunderstanding of how sub-groups work. While I've never looked closely at your particular mixer, if it is like virtually every other mixer in the world, your description of the subgroup assign buttons is faulty. Pushing a button (1-2 or 3-4) sends that fader to the PAIR of subgroups, either 1-2, or 3-4. (If the button is up, there is no send to a subgroup). The way to control how much of the signal is going to the individual subgroup is by PANNING. Panning hard left will send the signal ONLY to subgroups 1 or 3, (or both, if you depress both assign buttons; or neither if no buttons are depressed) Panning hard right sends that channel to either 2 or 4 (or both or neither). Intermediate panning positions will change the proportion of the signal going to the even and odd subgroups - straight up panning sends an equal amount to both.
So what is the use of a subgroup? Let's say you have a drum kit mic'ed with eight microphones. You bring each mic into it's own mixer channel, and using the eight faders and the eight panpots you mix the drum kit so that it sounds really good. But say there is a section of the song where you need to lower or raise the drum kit globally. It is hard to move eight faders simultaneously, especially if you want to keep the levels of the faders at the same proportion to eachother. Solution? Turn off the L-R assignment on each of the drum channels. (Now you should hear nothing). Now depress the Sub 1-2 assign buttons on each of those channels, and raise the volume faders on Sub 1 and 2. Make sure Sub 1&2 are assigned to L&R. Now you should hear your drum kit, mixed and panned just like the original. The difference is you can raise and lower the volume by moving just the two subgroup faders instead of the eight channel faders!
Hint: if you are using a dedicated reverb on the drum kit, you may want to assign the reverb returns to the same subgroup, so that the reverb fades up and down proportionally with the dry drum volume.
Other common subgroup applications: multiple tracks of background vocals, horn sections, and lead vocals comped together from multiple tracks.
Another advantage of using a subgroup is for processing. Most decent mixers have insert points on the subgroups. This allows you to use one outboard compressor to compress a whole group of tracks, by routing them to a subgroup (or subgroup pair) and compressing them as a group.
Remember, most of the time when you are using a subgroup, make sure the individual channels are NOT assigned to the main L&R mix, and make sure the subgroups are.
