The main issue is that the old SP system actually encouraged people to "play worse."

For example, you used to get SP for healing your party members. This meant that more damage received by your party members would result in more opportunity for skillups. Eventually people started playing "naked" to increase the damage they received, and mages would cluster close to the mobs to take damage from the AOE so they could heal themselves.

You also had a chance to gain SP for connected attacks. The SP was supposed to scale based on damage dealt, but the formulas were not set up properly. For example, Thms figured out that they would get more skillups by just spamming Phantom Dart than actually landing DoTs like Poison.

Basically the old SP system incentivized inefficiancy instead of rewarding cooperation and quick kills the way the Exp Chain system in FFXI does. Something just felt flat out wrong about the old system way beyond the "random" factor of skillups. Keeping mobs on "life support," dragging out every battle in order to maximize SP gain, and "cure sniping" are just a few examples, but there are many many other reasons why the current system is vastly superior.