That's good, and sensible, and everything; it makes sense when you read it. However, it took me - on about a year of experience developing for, administering and using Moodle - almost an hour to figure out what the problem was. Now, I don't want to be rude, but users - including teachers - are idiots. Things that seem flat-out obvious to you and me can be completely invisible to them. After all, the course might be in Visible Groups mode - you're right, the default is No Groups, my mistake, but the point still stands - they may not realise it. I didn't. I didn't create the course I was testing in; someone else chose Visible Groups as the default mode for whatever reason. For teachers that don't use Groups a lot, the Group Mode option is totally opaque, they simply won't trace that as the root of the problem.
So, this is essentially a silent fail. There's no bold "Some students were not allocated because they are not in groups ". There's absolutely nothing to tell the user why they are receiving this apparently aberrant behaviour. What's more, some users MIGHT be in groups, and other might not be, and the teacher could still have expected that everyone be allocated. If there's a lot of students, they're not going to notice until after some students email them saying they haven't got any papers to mark.
Or, let's imagine a real horror story: the student logs in, sees that they have no papers to mark, and placidly accepts this. They don't imagine for a second that they're missing out on twenty percent of this assessment just because their teacher didn't notice some aberrant behaviour in their LMS. The teacher doesn't see it, the student accepts it - after all there's no error, just a notice - and they get a lower mark, or fail their course, because Moodle failed to inform them of the situation.
Now this relies on both the teacher and all of their students not noticing this. But that's happened to us before, where each party thought the other knew what was happening. It's a big university, and a lot of stuff falls through the cracks.
Basically, what I'm saying is: when a user clicks Random Allocation, they expect all their students to be allocated. If they're not, YOU NEED TO TELL THEM, or seriously bad shit can go down.