Folks, thanks for the comments -- keep them coming! This was a somewhat controversial spell in our saga, so I was looking for feedback.
Let's separate comments about "game balance" from comments about "correct by the RAW". I will not address the former, except by noting in passing that there are far more efficient (and less dangerous) ways for a magus to operate in combat than simply swing a sword hard.
@jonathan.link @exarkun. Natural, slightly unnatural, or very unnatural? This was discussed extensively by our troupe. It's certainly no more than slightly unnatural, given that spells that make e.g. wheels turn entirely on their own have "slightly unnatural" as a Base (see the Hermetic Generans in TME).
I am using the Hermetic Generans as an example because:
a) it explicitly adds the material (metal) magnitudes to Base 2 (for Unseen Porter how this is handled is unclear).
b) a wheel that just keeps turning on its own (the Hermetic Generans) is more natural than an item that floats around on its own (the Unseen Porter), but less natural than an item that does not move on its own, and just moves when actively pushed as if pushed by a stronger but still "normal" person (Warrior's Wield).
@jonathan.link. Pommels and spearbutts. This is the reason why the spell works only on weapons that have a clear "grip portion", that's covered by the user's hands and thus not used to strike in battle. The pommel, or the butt of a spear, would not be covered by the spell. Now, you could argue that in some weapons with a clear "grip portion" you sometimes shift your hands around -- this is true for a number of medieval sword techniques, for example, which have the user grip with a gauntleted hand the base of the blade to deliver a forceful spear-like thrust. But I think this is pushing the discussion a bit too far. The notion that you can enchant only the "grip" portion of weapons with a clear grip portion comes from MoH, where a magus I'm too lazy to look up has just the shaft of his weapon enchanted, so he can use it bypassing MR, apparently at no penalty. This spell just follows that idea.
@jonathan.link @exarkun. Using a lightsaber is not trivial! One of the things that came up in our group was that this spell allows you to swing around really heavy stuff too easily to avoid accidents. Initially, we added three extra botch dice until the spell had been used extensively (the user had gained 5xp either in mastering it, or in improving his weapon skills while using the spell). But, really, if this is the case the same penalty should apply to any spell that boosts the user's strength or size. In fact, even if you have Single Weapon 6 (swords), when picking up a new sword you should not be as comfortable with it as with one you've practiced with for a few hours. So we decided against it, reserving the right to add a penalty on an ad-hoc basis if the situation warrants it (e.g. if the spell gets cast spontaneously, and the user never had any experience it with before).
@exarkun Why not Finesse? The answer is simple. In order to throw stuff with the sling of Vilano, you are directly exerting control on the magical forces; you do not need to move a single muscle. With Warrior's Wield, you are just setting the magical forces to accompany your normal fighting moves without really controlling them directly. In this sense, Warrior's Wield is far more similar to Gift of the Frog's Legs (ReCo 15 from the Corebook) that has the jumper make a Dexterity - Encumbrance roll to land without injury -- no caster's Finesse is involved. This is also the reason why you can really do anything you could otherwise do if you had a Strength of +5 in your sword arm.
