Alternate progression systems are a great way to go. Why add more stuff and make old stuff obsolete when you can improve upon what is existing? To me, this fits more of the mold of an RPG anyways. I have never liked getting new items only to replace them a few hours later. Building a character and being attached to them on their journey is fantastic. I feel that with the traditional leveling system and "more of the same" you end up having to come up with gimmicks. I am a fan of the Kaizen approach rather the Microsoft approach. Rather than coming out with a new version constantly, how about we just make the one we have better?
Because alternate level progression doesn't feel natural at all. Throughout the game we are leveling up. While there are some very rare instances of doing quests for bonuses (the Agent of Mara perk is the one that comes to mind), for the most part we become more powerful just doing whatever we feel like and seeing our skills increase and therefore our levels increase. This feels very natural. Raising the caps lets you continue doing what you do for longer and letting you still see character improvements. It is the most logical extension of the game.
@ adding new skills: Adding new skills is probably the WORST approach to raising the level cap. I don't mind new skills, but as a means to raise the level cap? Yuck. Double yuck. Triple yuck. I'd prefer alternate level progression a hundred times over forcing characters to pick up skills that they weren't necessarily created to use.