Randomly generated monsters whose stats use something like this would be neat as well.
It can be nice in games to become acquainted with the creep types. I'd imagine that too much random generation and use of sliding scales will create a game which the player is only loosely familiar with, because the creeps are different every time.
For example, coming up against a granite golem in warcraft 3, I know exactly how to deal with that. I know how many archers to make, and I know how the unit behaves; it's strengths and its stun ability and it's rock throwing ability .
Having a generic unit type with unspecific skills, is too much of guessing game.
Sure, have a sliding scale for creep levels. So a mud golem can be any level from 1-12; a rock golem any level from 8-24, a stone golem anywhere from 20-35, a granite golem anywhere from 30-50, etc. But keep abilities and behaviour constant within each creep class, just scale up the damage and hp.
Also, there can be an arbitrary 'elite' extension to a creep, which perhaps gives more magic reserves, or faster magic cooldown, an additional spell, or stats typical of an equivalent creep a couple of levels higher. But nothing too different. For example, in the golem family (mud<rock<stone<granite), an elite rock golem could mean that it is part of the way to becoming a stone golem, so give it one of the stone golem's spells, and increases it's stats equivalent to a rock golem +2 levels.
Also, calling oneself a mage or a paladin is a nice familiar title, and there is instant role familiarity, and an individual role to master, or to take preference of. Take that discrete classification system away, and what am I? Without a specific class, my identity falls back to racial identification.