It is bad for the same reason that everyone gets saves at a 1 per 2 level rate past epic: If you allow too much variance, you make certain saves guaranteed failure.
Example: Joe the Wizard boosts his Int by 20 from stat raises. All else being equal, this gives him +10 to all his save DCs. Assuming his spells had previously been about 40% successes (good saves for his enemies) and they started out with the same stat he put in Int in all three of Con Dex and Wis (a strong assumption!), that would mean before his 20 extra Int, they had a 60% chance to save. Afterwards, they have a 10% chance to save (on their good save).
Now, they have 20 stat raises also, but they have to divide those among all 6 stats. If they divide them evenly among Dex Con and Wis and ignore all other stats, they'll have 25%-30% chance to save against all Joe's spells. If they pump one of these, they'll have a 60% on that one and 10% on the others. If they dare to put up another stat, they'll be even worse.
But wait! This goes for Joe too. So Joe, by raising Int, will save at best 10% of the time against another pumped caster (and then only on his Will saves and only if he put equal stats in Int and Wis, which is unliikely). So the entire game becomes a duel of Initiative rolls.
