I dont think the name on the side would have anything to do with slots, just the number of seats. I know in DCA they will consider it as"regional" and let it use those slots-which is great for US. I'm not sure about LGA.
It would be worth emailing someone to find out the exact reasoning behind the decision. Ashby, maybe, or Baldanza may have an answer. When the aircraft was announced it was made clear that it would be a US Airways plane, dual class. This was reiterated several times in press releases. The first we heard of the Express branding was when someone posted a pic on here from the factory in Brazil.
One excuse could be that it's single class, which I dont think makes a plane Express. What if, hypothetically speaking, they decided to put 757s in an all-coach configuration for Florida services- does that make it an Express plane?
Its all politics in the end. They obviously decided that they'd like to contract it out and knew they couldnt let the contractors fly under the US Airways brand. Its also psychological, hoping the employees would be put in thier place- "your an EXPRESS worker now, just like that little plane over there, hence your unlivable pay etc." They also dont want active mainline employees paying too much attention to the plane- if it was mainline-branded they might feel more "ownership" of it, and god forbid start demanding the company halt its alter-ego airline and put it back on the mainline contracts.
They are wrongly assuming that customers dont care what it says on the side of the plane. A casual flyer might not, but the business folks that US so desperatlely wants do. With increased website bookings they do look at the equiptment (interestingly, the E170 flights currently DO NOT show up as US Airways Express on the website.) Its debatable on US markets that are getting an upgrade from 50-seat service, but if US starts wisely using the plane to serve new markets- say, SAT, or AUS, they might Express themselves out of the market. Or if they put it on "longer" flights- they could have an hourly E170 to ORD but all customers would see is that US only has RJs ithere now. No one wants to fly on an RJ, especially on a longer flight. When you have a plane that isnt an RJ, why call it one?
US has the luxury of having some of the finest airport facilities in the nation, and easiest connections, and this poor decision screws it up. In PHL, all US Airways Express flights depart out of the shiney new F Terminal. Ooops, except the 170 which tends to prefer terminal D but lately has been visiting C too. In PIT, all US Airways Express flights depart out of Concourse A or the E Shack. Except of course the 170 which favors the end of B.
As I noted above, it uses mainline flight numbers which prints "US Airways"- not US Airways Express on the website and boarding passes. They can get away with this because it technically is US Airways and they dont need to inform the public otherwise, which is required by law for other Express carriers. So they are trying to get around thier own poor marketing- the Express is for the employees only. Flight Attendants have been told they must wear Express wings and will be disciplined if caught wearing the US Airways ones they earned. These people come back to the company after several years of furlough at half the pay and a commuter contract and are told they cant wear the wings. Ridiculous. Talk about taking someones company pride and stamping on the last shred of it.