Originally Posted by Chicken
It's currently hard to say how good (or bad) expertise is; it's clearly the worst of the three from a threat point of view, but getting one of your attacks parried reduces the time until the next attack of the mob you were attacking, turning expertise into the only one of the three that affects your survivability; expertise is worth roughly 0.9% damage reduction per point of expertise.
Ratings needed for 1% hit with each different rating:
15.8 hit rating = 1% hit
12.6 spellhit rating = 1% spellhit
7.8 expertise rating = 0.5% less dodges, 0.5% less parries
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You said that it's clearly the worst of the three? Let's take another look at that. An attack which is avoided cannot trigger SoR, so it offers almost the same benefits as +hit at half the rating cost (and approx. the same threat benefits if you're behind the target or in front going past the -dodge cap).
Let's say you have a paladin with 3/3 Precision and 5/5 Combat Expertise, with no ratings from gear, against a boss with standard 5/5/10 M/D/P. That gives us this attack table:
6.00% Miss
8.75% Parry
3.75% Dodge
81.50% Hit / Crit / Glance (SoR procs regardless)
We approximate crits and glances cancelling each other out; this is probably not accurate, but as white damage is a small component anyway, it shouldn't be problematic. 20 hit rating will reduce chance to be missed by 1.27%, increasing SoR/melee/AS damage by ~1.6%. 20 expertise rating reduces chance to parry/dodge by 2.56% total, and thus increases SoR/melee damage by 3.1%. Take spells at 14% to be resisted, and 20 spell hit rating would increase your chance to hit with spells by 1.59%. This increases your threat from resistible abilities by 1.8%.
Now, based on some of my own WWS data on lone bosses,
roughly:
45% of threat is affected by Spell Hit
35% of threat is affected by Hit
30% of threat is affected by Expertise
So,
20 Spell Hit Rating increases your TPS by about 0.83%
20 Hit Rating increases your TPS by about 0.56%
20 Expertise Rating increases your TPS by about 0.94%