There are many reasons why someone would want to play an MMO. Below are some of the reasons...

1) To be the best of XYZ class on your server/game.
2) To have the best possible weapons/armor
3) To be the best crafter in the game
4) To be the best gatherer in the game
5) To play with friends in a team environment
6) To make new friends and help them
7) To grind, because you love to grind
8) To challenge yourself with figuring out new strategies
9) To try to do things others cant
10) To enjoy the detail that was put into the game
11) To max all your classes to the highest ranks
12) To obtain every skill/spell possible
13) To create your own strategies on how to beat things
14) To show off your e-peen
15) To achieve your next goals in character progression
16) To be able to make your own stuff (self sustainable)

I'm sure there are plenty more I left off and chances are you will have more than 1 of those in the reasons why you play an MMO.

With ONLY Horizontal Progression, you soon limit the amount of players types you are targeting.

With BOTH Horizontal & Vertical Progression, you have the ability to target every player type in the game.

If the vertical progression is too slow, you will lose players. So making sure vertical progression keeps at a constant, and horizontal progression fills in the blanks. This will lead to the most successful game.

You have to be careful with implementation of the new content to be able to reuse some of the old, as to keep it all important. This gives new players things to do, along with old players helping the new players or just doing it cause it was fun.