Interesting... Would you allow someone not to take the full value of his toughness? For example, Fae have have iron as their catch. If a Fae linked PC had 2 points of toughness then the catch would only be worth one point while if another PC had 4 points of toughness it would be worth 3 points. Would the two of them face different levels of iron use, or would the person with 4 points of toughness be allowed to treat iron as a 1 point catch (so he would pay 3 points for his power) and encounter it as rarely as the other PC?
To get back on topic, the difference between a 2 point power (Channeling, Ritual) and a 3 point power (Evocation, Thaumaturgy) is usually much greater than the benefits you get from a 1 point power. The power growth seems almost geometric, or closer to geometric than linear.
Richar