Depends what you're tanking and how you are tanking. If you are tanking regular crap in heroics and 5-mans, you only need 6% hit and spellhit to be capped (the protection talent lowers it to 3%). This is probably why you don't see much in the way of resists.

When you fight raid bosses, it's a completely different ball game. Just as with all other spellcasters, 16% spellhit is mandatory to max out your chance to generate threat. Whether or not you tank raid bosses determines the need to stack spell hit rating.

If you are still leveling your pally, try this: Tank an orange or red mob, either in the world or in an instance. You will occasionally see ret aura resist, holy shield partially resist, and seal of righteousness partially resist. Spell hit is what helps fix that.

Holy shield is my primary source of threat. In fact, on regular junk I don't even consecrate. The reason is that if I need to stop too frequently to drink, the rest of my party starts complaining about the lack of progress. You might use consecrate all the time for every mob, and if you do the logic is different... but if you just use holy shield and So/JoR it will be enough threat (9 times out of 10) to keep everything stuck on you, and spell hit rating helps keep holy shield and righteousness threat relatively consistent.

In conclusion, pally tank itemization is complex and almost everything that you think might help helps.