The thing about hit rating is that you're trying to minimize the RNG of missing with your attacks. You could miss with a filler spell, like Fireball or Frostbolt, and it wouldn't matter as much, or you could miss with a Combustion, and cost you a chunk of your dps, as well as waste your cooldown.
By hitting the cap, we're eliminating the RNG that's associated with missing hits. Sure, as Kenbud says, you could be at 14.80 hit and still never miss in a fight. It is also statiscally possible that you miss 3 or 4 hits out of 100, and if that happens to be one of your big spells, you'd lose out on dps. It's why we try to stick as close to the cap as possible.
Anything over hit cap is also wasted stat, which makes balancing it a tad difficult. In the end, you'd have to judge for yourself if 4-17 rating of secondary stat (this equals around 0.01 - 0.05% hit) is worth being over or under the cap.