I do agree with you. The problem if you want to call it that comes down to some players perception of it.
I've posted this in other forums for ARPGs such as D3. I think a lot of players get caught up in a "Viable" vs "Efficiency" dilemma.
Group 1 views it as such...
Viable = Can complete all content
Efficiency = Doing the content the fastest/most optimal
This group tends to be ok with varied time/effort put into achieving said goal....not to say they would be ok with option A taking an hour longer than option B....but they can live with "less than optimal" play as long as it can finish the task within a "reasonable" time frame.
Group 2
Viable = Efficiency
If option A isn't as efficient as option B...then option A isn't viable and hence the game is imbalanced.
I guess my point behind this is this seems to be where the real argument is hiding within and depending on which group you fall into...makes the attribute system look quite different.
Power gaming can be fun but appears to be a sickness for some people.
Whether you use PoE's system or a more D&D-like system, power gamers will still do their thing, so we can safely ignore them.
In terms of game design, the problem of balance can be approached, I think, from two axes.
The first axis concerns equality vs. equivalence. Some view balance as being something where everything is equal, i.e. no class does more dps than another, no build does more dps than the other, and the differences only affect non-mechanical or story aspects. Others view balance as equivalence: while one class might do more dps, another will provide more utility, and while some particular build might be weak in some areas, they will be strong in others. Most RPGs tend towards equivalence due to the number of different kinds of abilities that cannot directly be compared against each other.
PoE is no different. In fact, since it is a story-based singleplayer-only RPG, it could do with even more divergence here. I think the devs focus too much on combat balance as opposed to overall balance. For example, providing some optional abilities for Fighters that can counter spellcasters is good, but making it so a group of generic Fighters can take on a balanced party is bad. In a group of only Fighters, some should be required to specialize in dealing with spellcasters, some with ranged attackers, etc. in order for that group to even be able to scratch a balanced party. And making those different possible builds equivalent in utility and power is good, but making them equally good overall is bad.
The second axis is about symmetry vs asymmetry. In every RPG, there's a needed amount of asymmetry required as you introduce classes. Another source of asymmetry is usually the character stats. This is not the case in PoE though, where every stat affects every class in the same way. Personnally, I think having more asymmetry is better. But that doesn't mean you need to make stats only affect certainly classes or types of abilities. The only problem I have with the character stats in PoE is that they affect all classes in the same way. If stats affected classes in different ways, I think it would make for better build variations.
For example, maybe intelligence gives AOE to all clases, but for Wizards it also gives more spells and for Fighters it gives a bonus to attacks of opportunity or even simply a small overall bonus to all combat rolls (a more intelligent fighter is overall better than a dumber one). And maybe some stats give no direct advantage to certain classes. For example, Might is utterly useless for a Wizard as far as his Wizard abilities are concerned, but a strong wizard can fight better in close quarters and can carry more **** (so you might want to introduce actual significant weight for multiple grimoires, scrolls, potions, and maybe even spellcasting materials). Wizards that can unleash godlike spells while naked are all well and good, but there's no reason while it needs to be that way. So maybe Might/Dexterity changes how armor influence spell failure. So a beefy, highly dextrous Wizards might be able to cast spell relatively well from inside full plate armor. This way you could use touch-based spell more safely, or you could use defensive spells to tank up and let your Rogues backstab everyone, etc.