I asked the same question a while back. The replies I got were mostly that in some models, in some cases, the FLGR adds balance and muzzle weight. Perhaps I'm a tasteless philistine, but I can't tell the difference.
Some have said that IN THEORY, the guide rod prevents the spring from bunching and/or twisting. But I have never been able to find a case where this has actually happened.
I frequently field strip, not just for maintenence, but also because I shoot a .22 conversion kit. Besides already having pock-marks in the ceiling from losing my grip on the plunger with the hole in the middle, there came a time when I was in my dad's pathfinder, using the lip on the front edge of a magazine to hold it in while I was making the switch, and I thought to myself: "I'm going to be in this car until midnight looking for this stupid part, aren't I?" I have recently decided to build a dedicated frame for the .22 anyway.
Yes, it makes it easier to 'press check', but think about it, press checking is breaking the rules anyway, putting your thumb in the trigger guard. (I'll admit I still do it sometimes.)
I was watching Tom Gresham's Personal Defense TV the other night, and Clint Smith was giving a class on running your gun with one hand. He walked through the process of reloading and clearing stoppages with one hand for rifle and pistol. If you have to charge a pistol with one hand, this pretty much means putting in a magazine, putting the front of the pistol on the edge of a hard surface, and pushing forward to force the slide back to charge it. If you have a pistol with no guide rod, you have to make sure the barrel is clear in the front to do this. If you are using, well, pretty much any other auto, you have to make sure the guide rod AND the barrel are clear in front before you push forward. It might be a small distinction, but if you are in a position where you have to do this, it probably means you are already wounded, and every detail you don't have to worry about helps.