A lot of the disagreement is coming from the fact that some are talking about a hypothetical Ovi store that (a) has decent apps in it and (b) works, while others are talking about Ovi as it is. Attila is talking hypothetically, and is right. As things are now it they might as well merge the lot (for all the draw that Ovi is), but in a case where Ovi has good offerings and is therefore a premium service (rather than one that charges a premium for no service) it would indeed be a loss to the devs (and, i would say, the users) to combine everything for the reasons already given.

Edit: A separate consideration given the actual state of Ovi, is whether we want it, meaning whether it would be a good idea to group it with the Extras repo of tested software. It has proven multiple times to house flaky, possibly dangerous software that would (theoretically at least) never make it into Extras.
Small example just coming up now: http://talk.maemo.org/showthread.php?t=47215
Also see the lack of optification of Ovi apps.