The real value of registering officially is participation in the formal testing surveys. It's in Wizards interests that groups playtest these rules, and if making that happen means they do not capture 100% of the players in those groups, that is an acceptable social overhead cost to getting as much feedback as possible. So I agree the RAW of the OPTA works against them (as well as presumably for them, in their evaluation), but I think they are counting on folks being practical about these things.
If that is all it was, I'd agree. However, it is more than that. This skews the playtest to people willing to jump through some specific hoops that don't really have anything to do with giving the rules a good try, and then giving feedback on that. Moreover, it means there probably isn't any provision to collect group feedback through the DM. (I don't know for sure, because I couldn't get the packet yesterday, despite having signed up the week it was first announced, and trying for a couple of hours last night.)
I'd have no trouble getting my group to really pay attention, play the game as it seems to be intended, think about it, and then give some feedback. We
like dusting off rules systems that way. But I don't think I can get them to navigate the train wreck that is frequently WotC's website. So the universe of playtesters turns into the super set: People willing to playtest the game
and people willing to jump through some specific and annoying hoops that have nothing whatsoever to do with the game itself.
Now maybe in the early playtest, that is an acceptable trade. Maybe when it comes to dealing with the heart of the system, you want a certain amount of arbitrary hoop jumping as a self-selection method. However, if this continues for the life of the playtest, then it would be like testing, say, the usability of a new remote control layout by limiting to people who can change the oil in their car.
Perhaps they will open up, loosen up, as the playtest goes forward. Maybe "group reactions" are more important when you have a wider range of material to test. I don't know about the "when"--only that eventually it becomes important.
Also, the cynical side of my nature can't help but see the clumsy hand of marketing wanting to get email addresses to push stuff.