And you must remember that MorphOS is perfectly usable (without any limitations whatsoever) without a registration, at least for 30 minutes at a time... 
I do not doubt that there are some people who have sampled MorphOS 2.x without paying for it, but you should be careful with making too much of this, as you are likely to undo your own argument. For example, if we class 'OS samplers' as users, then think about what this means for AROS. It's possible to try it out in a VM without too much hassle, which many people have done (not just across the whole Amiga community, but those interested in alternative OS too). Furthermore, AROS is now included in WinUAE, and there will come a time where it'll be bundled with all versions of UAE. Simply put, MorphOS hasn't got a hope in hell of competing with the numbers of people who use UAE, not in its current state anyway, so I wouldn't make too much of the '30 minutes at a time' users.
There seems to be some sort of general consensus that "a fair number of users" would currently be obtained by dividing the registrations with 2, and I agree, I think it's a fair assumption. This would give around 700 MorphOS users, which is a pretty decent number actually! 
Okay, let's say for argument's sake that there are 700 people who have at least one registered copy of MorphOS. I still think this is somewhat optimistic, but let's use this figure for now...
I *sincerely doubt* that any active OS4 user haven't downloaded this piece of software by now
Then your sincere doubts have done you a disservice. Not every OS4 user can run Timberwolf, and I've heard accounts of OS4 users saying they're not going to bother with it until it's more mature.
I don't want to belittle AROS in any way, but MorphOS and OS4 plays in a completely different league.
I am under no illusions that AROS has fewer regular users than OS4 and MorphOS, but that doesn't concern me, the momentum for AROS continues to build, it's not a question of 'if' it'll catch up/overtake the other two, but 'when'.
About 700 active MorphOS users would mean 2x the amount of about 300 active OS4 users coupled with some 50 active AROS users.
But again, your guess is as good as mine...
That's your guess. Okay. Let's look at it another way. How active are the following forums:
http://www.amigans.net/http://www.morphzone.org/modules/news/http://aros-exec.org/At the time of writing, 7 topics on amigans.net had posts from 'Today', 2 topics from morphzone.org had posts from '11-03-2012' (today's date), and 9 topics from aros-exec.org had posts from 'Today'. Your definition of active users may be different from mine, but seems unlikely to me that MorphOS users are that much quieter/less opinionated than OS4 and AROS users, seems much more likely to me that there are just fewer users who are actively involved with MorphOS. YMMV.