I cannot comment from a tower perspective, but from a center perspective in which we blend traffic/start spacing for almost every major airport in the central to eastern US, here is one perspective:
Facts-
1. Southwest almost always flies at or very close to their maximum forward airspeeds. They rarely pull back to conserve fuel. They don't pull back in the climb/descent unless asked to. This usually makes them faster. Faster plane in front gets there quicker.
2. They ask for shortcuts more than any other pilot group. The more you ask for a shortcut the more you get them(unless you pester, basically ask for it once per frequency change, eventually you'll find out who can authorize it).
3. They rarely ask for impossible short cuts(i.e. direct MDW when landing at MDW). They fly into Midway all the time and seem to remember on the Fissk 1 arrival they are only going to get Kokomo. So what do they ask for? Kokomo. Ask for Veeck especially durring a push and there is an answer I can definitely tell you that you will get: UNABLE. Depending on the controller you get and their workload, they may take the time and give you kokomo, but that depends on the controller. I can't tell you how many times I get pilots asking for Drakk on the Erlin5 into Atlanta durring the middle of the day when they know all we can give are BNA or BWG. Also memphis arrivals constantly ask for LTOWN or WLDER when the best we can give is SPKER. Do we as controllers expect you to know all this? No, but if you fly the same arrivals every day into the same major airports, ask and remember what you can get. I know everytime I fly, I try to talk to flight crews that go through my area alot and give them tips on what we can and cannot give.
4. In slowing an aircraft down to let another aircraft in front...... To do this soley by company would be somewhat impossible, if not dangerous. My sectors constantly average 8-12 aircraft at a time with 20-25 durring peak times. To keep from falling behind, a good controller can only do two things to sequence aircraft: the faster guy stays in front or the closer guy is in front. Meaning in a distance tie the faster guy is definitely going first. In a speed tie, the closest is going first. Speeds, vectors, and shortcuts are given in respect. Now there are more factors we could spend all day talking about, but that is the simplest way to figure it out. Also the closer you get to the airport the more distance plays a factor and the more likely you are slowed down. If you rearrainge the sequence any other way your going down the craphole faster than Alice went through the rabbit hole. The only way you would risk that is if there is a priority(i.e. emergency, lifegaurd, possibly min fuel, etc). Nowhere in the 7110.65 or any other regulating document have I seen Southwest listed as a priority, therefore I don't know a single controller who would risk his career for southwest.
Last, I would like to invite any U.S. citizen to come take a tour of our facilities(U.S. citzenship is a required by the facilities). Find a controller or get a phone number and ask. The worst anyone will ever say is no. If you want a tour of Indy center, shoot me a PM and I'll do what I can to get it set up for you.
Hopefully this gives a controller's perspective on the issue.