Also, OP: The main problem I see with it is that there are various ways to get extra move actions each round, while there are hardly any ways to get an extra action of any other type in 3.5.
Two ways that pop to mind at the moment: Marshals have an aura or something to give allies extra move actions IIRC, and the Hustle psionic power (psychic warrior, 2nd-level) gives an extra move action (at the cost of a swift action though, so it would basically balance out in this case, but I dunno about other cases or other sources of extra move actions).
It would also allow spellcasters to use 3 spells per round; 1 standard action, 1 Quickened as a swift action, and 1 Quickened as a swift action that replaces their move action for the round. That was the main reason Haste was changed in 3.5 to no longer give a free partial action each round; it gave spellcasters too much firepower per-round (and likewise for psionic manifesters). With metamagic rods in the 3.5 core rules now, IIRC, it's not too hard for a mage to get a Metamagic Rod of Quicken Spell to help with this rapid-fire assault strategy.
It's a 50% (or maybe more like 20-35%) power increase for spellcasters/manifesters, since they'll be able to use an extra Quickened spell per round (though not manifesters, in 3.5, since 3.5 Quicken Power requires expending psionic focus). The 20-35% part is just cuz one or both of the Quickened spells will be using the metamagic feat, so they might do somewhat less damage being lower-level spells (i.e. Quickened Fireball instead of Delayed Blast Fireball at 15th caster level).
