A lowboy is a nice way to pack a descent sized screen in a very small form-factor that is not "toyish". I think it's hard to get a better screen size / cabinet size relation without going for an LCD.
As for the marquee, yes, it's mostly a aesthetic/nostalgic thing. But that doesn't mean that you should ignore it. As lowboys generally (not your case) don't have sideart, the marquee is what defines it. Having different marquees and changing them from time to time may be a way to keep your cabinet fresh. Maybe now you're into classics, but next month you'll be more into fighters or shmups... Your cab may change with you

.
On a more practical note... While the marquee itself may not have any practical reason on a home arcade, the space it creates with the inner side walls of the cabinet helps to isolate your peripheral vision a bit, it makes the game more absorbing, you get less distracted by what's around you. It also provides, in my opinion, a better place to mount the speakers. The sound is right in your face, much more connected to the game.
As for the Urban Arcade, I'm sorry, I think it's a horrible design. The main thing I don't like about it is the angle of the screen/glass. If you're going to have such a low angle, why not just make it horizontal and have yourself a nice little one sided cocktail? At least you'd have some place to rest your beer

. You could probably keep the screen turned toward you and make just the glass horizontal.
Don't get me wrong, I'm all for new and non traditional designs, It's just that that one doesn't cut it for me. If I were you, I'd either go with the traditional and proven lowboy (thanks for mentioning mine, BTW) or would come up with my own thing, based on critical approach of various designs. I'm pretty sure you can come up with something a lot better than the UA.
Hi again
I was doing some research on cabinet shapes etc for a small form-factor cabinet. The front-runner and current plan is to build a cabinet based on an Aussie lowboy, like what Holdennut and hbm*rais both built:
http://forum.arcadecontrols.com/index.php?topic=54567.0
http://forum.arcadecontrols.com/index.php?topic=28763.0
But in my research wanderings, I've also discovered an Aussie company that builds arcade machines that is of their own unique design.
http://www.blacklight.com.au/urban/cabs.html
The biggest thing that stands out is that these machines don't have a marquee at the top, so they mount the speakers at the front.
For some reason, I like this design. Its very clean. Except it doesn't have a marquee. But since I plan on Knievel side neons with a MAME logo, everyone will still know that my machine was a MAME box. So, strictly speaking, I wouldn't need a marquee at the top announcing what it was.
So, does the marquee have a purpose other than (traditionally) advertising what game it is playing? Is it functional in terms of shielding the screen from overhead lights and glare etc?
What do people think of the Blacklight marquee-less design?