While it's pretty dang early to speculate, if this will be even remotely like an Orlando hotel, a Universal owned hotel would be able to offer perks exclusive to the park (possible things: early entry, included Express pass benefits, soft drink refills, discounted park tickets, and special packages if so inclined), as well as higher quality control, service, and most importantly, theming. You can't tell me the vibes of Cabana Bay or Royal Pacific are on par with a Sheraton that looks like a box.
And who cares if USH isn't a full day park? Doesn't mean the guests have to exclusively go to USH... there's craploads of tourist destinations a very short distance away: Walk of Fame, Hollywood Bowl, Chinese Theater, Hollywood & Highland, Ameoba, Griffith Observatory, etc. Sure, guests could do all that from other hotels, but not with the added benefits of exclusive USH benefits, quality, and convenience.
As already pointed out, Knott's is not a full day park either, and they have an onsite hotel that's doing quite well--even though it's surrounded by tons of other hotels all along Beach Blvd.