When any skill or talent in WoW reads, "...increases the {statistical} chance of {anything} by x%," what that means is that it is increasing the already existing chance by x% of that chance, not directly adding x% to that chance. The property of this phrasing is multiplicative, not additive.
To explain myself, let's say your Thunderclap was at a 5% crit chance before you spec'd into Incite. The math on Thunderclap with Incite would then be:
current crit chance + (current crit chance * .15)
or
5 + (5 * .15)
or
5.75
|
In this case, your crit chance only went up by .75% (not even a full percentage), and you used up three talent points to get it. Before Incite is worth anything to you, I should think you would need a significant amount of crit chance percentage.