So I think the idea is that the bonus from your pyrokinetics multiplies with your int bonus. So for example, If you have 20 int (50% damage bonus) and 5 pyro (25% fire), on a 100 damage attack, you'd deal:
(100 x 1.50 X 1.25) = 100 x 1.87 = 187 damage.
Whereas all those points put into polymorph and into int would give you simply:
100 x 1.75 = 175 damage.
With 30 int and 10 pyro, you'll deal 2.00 x 1.50 = 3x more damage, rather than 2.5x more damage with pure int.
Hence, going into both pyro and int is better than pure int. This should be the same with huntsman and dex, the 5% bonus from high ground multiplying with your dex bonus (AND your weapon bonus, I believe).
If I have this correct (someone verify please), Larian should make this more clear somehow. On first glance Pyro does seem completely inferior to intelligence, but when understanding it can actually add more damage, it makes more sense.