Yes, yes it would, as an offline player, the lack of instant action options, especially when it comes to bots, is simply not good enough, alot of the stuff i used to do, that i had years of fun with in past UT's, is just not there, and it means i choose to play UT2004 or something else instead.
Also, and lets be honest here, most of us have a bunch of good games we like installed on our systems at any one time, and the more annoying and problematic a game, or part of it, is to play or work with, the more likely we will choose to launch a different game instead, especially if you just want a quick casual play after a hard day, you do not want to mess around with such things, you just want to get in there and enjoy yourself, and this definately means that a bad UI will make you play it less, as you put it aside for when you have the patience to put up with its annoyances.
But the real kicker for me is the lacking options, sure the interface is annoying and slow, and the game wont remember anything i tell it from one launch to the next, thease things do bother me a bit, and they do make me play a bit less when i am in casual mode, but it is the missing and broken things that really ruins UT3 for me, because thease things ARE gameplay, they DO matter even after i have left the UI and find myself ingame, and they dictate with whom i play, and how.