In my experience unless a carrier is self handling it can make little difference. At my local airport everyone is handled by one Pax agent and while airline policies (LRV's etc) vary, the same staff will be handling everybody from FR through the charters to KL and other full service carriers. I think the scenario described above is more a matter of generic staff shortage and unwillingness than rudeness due to the fact that the agents are representing a Loco at that time. The idea that the same staff would have helped a the passenger if they were with another airline is not realistic in my view.
As an aside, despite all being from one company with us EZY have their own dedicated handling agent staff and in my experience they are possibly a little more useful than the mainline staff. This may be due to them being able to focus on one airline rather than having to represent different ones all the time. I do agree that you may come across rude and unhelpful pax handling staff, but this is not limited to LOCO's by any means.