Yes, Polymorph using Strength does make sense in the context of the theme, but that isn't entirely necessary.
Intellect is often associated with Magic of all forms, with only a few occasions of magic being governed by other areas (e.g. Paladins in WoW primarily using Strength but still use Holy Magic, and Enhancement Shamans using Agility, since their spell power is half of their attack power). It really depends on the Class and Skill Tree.
However, hybrid systems are far from irrational, as they have been done before. Many MMOs and RPGs have had classes that can fill multiple roles based solely on their gear composition and skill set (some classes can be both healers and tanks). Some MMOs, such as Champions Online (the example for the next bit), have Stats that govern very specific things, e.g. Strength increases your Physical Power, Dexterity increases your Crit Chance and Power, Constitution increases your Health, Intelligence affects your Cooldowns and Energy Costs, Ego improving Ranged Damage and Hold Resist, Presence increasing Healing and CC Resist, Recovery improves your Self-Healing, and your Endurance increases your Energy Thresh-hold.
In Original Sin, this is handled differently: Strength increases the power of Strength-Based Weapons (swords, axes, clubs, etc.) and your carry weight; Dexterity increases you Defense Rating (surprisingly) and the power of Dexterity-Based Weapons (bows, crossbows, daggers, etc.); Intelligence increases the power of Intelligence-Based Weapons (staves and wands); Constitution increases your Maximum AP and Vitality; Speed boosts just about everything AP and Movement related; Perception boosts... well, detection and Critical Chance, and reduces Ranged Hit Penalties. NOTE: Strength, Dexterity, and Intelligence all reduce the cooldowns of abilities affected by them.
In all honesty, if any Attribute fits the bill for Polymorph, Dexterity seems to be the best fit. YES, Strength can easily be a factor based on the context, and it would make sense if the spells in question increase your physical strength. However, another argument could be said for having it governed by Dexterity. I can imagine that Polymorphing on a moment's notice could require a bit of finesse. And Dexterity is often associated with finesse. I'll leave it to you guys to figure out how that might work...
