With the additional Haste modifier, the proc rate is already higher for specializations which stack a lot of Haste, but becomes even greater still once Overwhelming Power itself procs. As the buff increases haste, and therefore the proc rate, it effectively double dips, leading to a rather high uptime and chain procs, often refreshing itself before the initial proc fades. Specializations such as Fury Warrior and Havoc Demon Hunter often achieve 50%+ uptime, and this change reduces that to a more reasonable ~35%.
Overwhelming Power remains very competitive, and perhaps even the best trait for specializations which value Haste, but should be closer in line with other traits, rather than
in a league of its own overwhelmingly powerful.