Without taking RPPM procs into account, sacrificing the expertise hard cap to increase haste is a theoretical decrease in Holy Power generation and dps over the course of a several-minute-long fight. This drop becomes more and more severe the more haste you have on top of what you're gaining by giving up expertise. "Reliability" need not enter into the discussion.
For each 1% haste you gain in preference to hard-capping expertise, you're losing 1.25% expertise. Even if these two values were identical, the drop in expertise would still always have a greater negative effect than a concurrent equal gain in haste.
Code:
Exp Rtg Hst Rtg Hit % Atk Spd Throughput
2550 2550 92.50% 106.00% 98.05%
2600 2500 92.65% 105.88% 98.10%
2650 2450 92.79% 105.76% 98.14%
2700 2400 92.94% 105.65% 98.19%
2750 2350 93.09% 105.53% 98.24%
2800 2300 93.24% 105.41% 98.28%
2850 2250 93.38% 105.29% 98.33%
2900 2200 93.53% 105.18% 98.37%
Given that, then considering that losing the hard cap also exposes you to bad-luck RNG streaks, I can't find any reason to gamble for possible (but uncontrollable) short-duration gains at the cost of overall performance in both HoPo generation and dps/threat.
[Edit: Firefly, you make a very good point (as always) that many of our attacks are spells and do not benefit from expertise beyond what is necessary to achieve 15% spell hit. I'm still torn on the overall effect on HoPo generation with our three main holy power generators being subject to parries -- and our fourth generator generally being a proc from either of those three except on AoE pulls. For dps/threat, haste almost certainly prevails.]