Well, as leveling increases you'd have to increase our characters total power. This leads to some of the issues we have with current content, being many older pieces of content can be easily skipped. As it stands now, there is little to no purpose of doing the third dungeon because as we level up to 50, the gear obtained from there is easily outclassed by the higher level easier to obtain green items of the foot hills.
Essentially, creating content for a capped level productive becomes much easier to balance, and allows for new unique abilities to be put on new gear that otherwise might be over or underpowered if it came on lower leveled stuff.
Which is where the GoH Master Point system comes into play. That game allows you to level past the level cap, without giving the benefits of increased skill points and such. This means that a 10 master point item (which would essentially be level 60 in our case, level 50 + 10 master points) would be much weaker than a 40 master point item (which would be level 90

). As it stands now, we'll have a ton of level 50 items with no real way of differentiating them, and with no real way of feeling like we're progressing from trash mobs.
Allowing us simply to level up straight to level 100 would drastically affect how most of the formulas work. We've already had massive issues with how crit% was calculated based on an old formula by the time we got to level 50. If we were able to continually increase that, nearly every character could potentially be near 100% crit chance and that's not an ideal situation.
So I think having a system that lets us "Level up" without powering us up would be a good addition. It differentiates pre-d4 gear that we'll be getting from actual D4 gear itself, and it gives us some reason to feel like we're progressing from killing trash mobs instead of it feeling like a pointless time sink.