Top 2 reasons I would assume are:
Slows the pace of *official* upgrades for those models (not that anyone is satisfied with the pace of upgrades for SenseUI or stock phones either) and ZOMG it has those widgets for the kiddies to keep up with facebook, myspace, twitter etc whatever else it supports oh noees! I think they assume they lose their "tech cred, yo" if they admit liking something like that.
I used the Devour with it for about 1.5 months and just switched to the Droid this week. I immediately tossed the widgets but it seemed to have quite a few improvements over stock IMO. Now how much is tweaks for just that phone, or Motoblur in general I don't know, but the ones that stand out are:
The phone/contact tabs integrated with the app drawer tab (it seems froyo may take care of this by default now as well, it has phone/browser integrated)
The account integration: I had my gmail, facebook, yahoo, hotmail and twitter accounts set up through it. It had more flexibility regarding the contacts than stock seems to have (you could link multiple contacts into one contact, not sure with stock as it doesn't have many accounts supported anyway) and after a factory reset all your accounts were restored as well

Contact list had a little tweak I liked, being able to set up different groups was very nice (so I could just choose the "Family" group for example instead of scrolling through all the contacts)
It had a PIN unlock option as well pattern unlock (froyo has pin and password unlocks)
It also had separate timeouts for screen as well security. This meant I could have screen timeout in 1 or 2 minutes but I wouldn't need to do the security unlock unless it had been idle 20 minutes. I liked this a lot.
Not sure if it's a 1.6 vs 2.2 browser thing, but I liked the fact that the "windows" selection showed me little thumbnails of the windows instead of just the names...
Looks like the new version seen in leaked pics has themed or less obtrusive tabs on the home screen than the blue/green version in the existing phones. IMO it actually adds quite a bit to Android, and you can bet on a phone that will probably be as popular as the Droid Shadow (so much better of a name than XtraXtremeXtraXcitingXfinity blah) getting rooted and hacked. I don't think it's as up front and center as SenseUI in what it adds...more of some underlying tweaks and enhancements than outright UI changes. Outside of the blasted widgets lol.
That's my 2¢ anyway, hope it helps.
edit: it also backed up contacts to your Motoblur account (good in case of extreme Google failure) and you can locate and remotely wipe your phone through your Motoblur login. I know lots of this stuff can all be added with apps/addons but it is nice having it all there from the beginning.
double edit: in this case on VZW though, they also locked some stuff down to their crapware, so for instance if you opened a contact and wanted to IM their yahoo email, your only option from there was their stupid Mobile IM program (which uses your text messages--I only have 250/month--instead of the damn $30 data package). No option to switch. Same with navigation/maps--if you had dutifully entered in the address for a contact but then tried accessing it through the contacts, you were only presented with VZ Navigator as an option (for $9.99 more a month). Now you could go directly to Google Maps/Nav and then select a contact from within the app but to me it seems the contact list should be an easy to use and central point when dealing with your contacts. Couldn't uninstall their crap either, that damn Visual VM was always effin starting itself and running (NO I DON'T WANT TO PAY $2 or 3 bucks a month for it VZW!).