Heel-toeing is lot's of fun, I can't speak for doing it while using a G25. In a standard shift street car I can however. As others have said, it makes a downshift between two gears seamless, the drive line never receives a shock of power, or sudden braking that will upset your rear tires, probably front tires too, but I've never done HT in a front drive car. Not to mention you are never having to lift from the brake or accelerator pedal, keeping the car under full control.
After some time it can become second nature, just doing it under normal driving, however if you find an empty back road to push on, it makes it all the more fun. Besides all that, it's one of the best noises a car can make when your rev-matching, well aside from flat throttle up shifts that is. That however serves no purpose in city driving, and can lead to a loss of traction if your not careful. I've only used it for drag racing, heel-toeing I've used for commuting to work, not necessary, but it sure if fun.
Do open throttle up shifts gain you anything with a G-25? (Accelerator to the floor, pulling against the shift handle pressure, kicking clutch which releases pressure slotting into the next gear. Do the shift handles even lock up like a real car under power or can you just gently pull it out?) I don't have one, but it would be interesting if you could do that and gain time seeing as how others have said heel-toeing serves no purpose. It would be even cooler if you could screw that or heel-toeing up in GT5 and screw up a gear in your transmission, that could lead to taking some care during shifts.