It remains a useful list of differentiation, and works just as well across game types as a means of quantifying power. Up to T4, its a simple matter of power, above that its a measure of versatility.
Of course, the actual metric isn't exactly linear, things do work in two axis after all.
Power's metric cuts off at T4, but it has a similar spread(just that +infinite damage is the same as +enemy health damage) in practice. You can say that T4s have enough power to solve a problem(namely a fight), that T5s do not have enough power to solve a problem, but theres no measure for having power in excess of solving the problem, and power in massive excess on their own.
Despite my conflicts with Jaron, the concept of tiers is sound. Just he draws the dividing lines in the wrong places, for the wrong reasons, and comes to the wrong conclusion based on those tiers. 3 up is primarily defined by what it can do. 4 down is primarily defined by what it cannot do. The correct conclusion here is to stick to 3 up, not that anything works in the right game.
As for the lines between tiers themselves, 6 essentially can't do anything. 5 is either a one trick pony or those who can't do much (only not in 6 because 6 is the NPC class tier). 4 is the better one trick ponies, as the ones that try to do more than one thing here still don't work that well. 3 up you start encountering diverse characters.
In terms of specific entries, skills are overvalued by the tier system, and the lines between classes at 3 up are way off. If it's based on 1s having plenty of TO material, 2s having some and 3s not having any that's not true as there is plenty of TO to go around. If it's based on spells Beguilers have most of the same offensive and defensive options as Wizards, TO material excluded. Conversely, Duskblades have no special defenses and only damage for offense and Warblades aren't much better. If it's based on access to game breaking tricks anyone can get most of those if you're counting them as a valid metric.
As a short summary though, saying higher tier = more and better spells is generally accurate. Likewise, skills being low tier is also quite accurate, assuming that they are all you have. If not it's just a non significant part of your character.
Just look at what characters do. Now a Warblade is going to be a DPS character, same as many others. What differentiates him from his other, lower tier brethren is defense and support maneuvers so that he is somewhat less squishy. He is also better able to get off full attacks due to Sudden Leap, etc meaning he can better use the tools that he has.
A Beguiler is going to have a bunch of different save or loses. Many, but not all of these are mind affecting. They're also going to have an array of defensive spells. In short, they're not going to play that differently than a Wizard in combat, except that they have a better base casting mechanic, and they're not as good out of combat.