Just so we have a single post to refer to this single point:
A modern MMO has skills that scale with level. You acquire the skill once, and its numbers increase with you. If your game has something like Fire Bolt I-VII or Lesser/Moderate/Greater Sudden Strike, it better have come out before 2005.
See City of Heroes (note: 2004) or The Lord of the Rings Online™ for how to do this properly. See Asheron’s Call or World of Warcraft for how to be old school. See Dungeon Runners for an attempt to cut the difference.
Having single skills that scale with level allows you to do reduce some of the harm caused by having levels. See sidekick, mentor, and exemplar systems across various games. Having many levels of skills hides the fact that players are not getting anything new this level while creating hell levels and erratic power curves.