Why do you think it's only viable to put combat ability points in Polymorph?
With a point in Necromancer you get for example more self-heal through damage. A point in Warfare ups the damage against physical armor by 10% which may be better in some occasions than a general increase of damage by 5%. A point in two-handed gives you 5% damage and some crit chance.
A point in Polymorph gives you an additional attribute point. Which is nice of course. But with the second benefit of attributes (for example + physical armor for Strength) removed now, you get not more or the same than from a point in combat abilities, but less. It may be a good decision to put points in Polymorph for an Intelligence based build.
Then there is the rumor that higher skills need several points in the according combat ability. Which is in my opinion absolutely necessary for the health of the game, as it's biggest fault is the easy way to amass ubar skills from several schools.
