Hit and expertise do matter.
I've heard both sides of the argument -- that you don't need hit and expertise, where others say that you do. I'm firmly in the 'need 'em' camp, because I work with DPS who go all in and all out from the start of the pull, and every time you miss, parry, or get blocked/dodged when they don't is a chance for them to gain aggro on you. I've had pulls where I was stacking stam trinkets (losing both hit and expertise since I have a reforged Fluid Death as one of my trinks) on early attempts on a heavy hitting boss (Onyxia at 352 gear level, a month or so back) and at one attempt, my initial FFF missed, my initial mangle got parried, and my initial white attack got dodged. Meanwhile, the rogue had pre-potted Agility, opened up with a Sinister Strike that crit, and whoom. Ony was his special friend.
My housemate also has a bear tank. She got new bracers and a new weapon upgrades one day, and lost 10 points of Expertise by switching them in. Suddenly people were pulling off of her when they weren't before. Switching back to her old bracers and weapon for the remainder of the night solved the problem temporarily until she could reforge for the expertise.
The main argument for 'you don't need it' is that 'get Vengance stacks and nothing can touch you on threat.' The key problem is that you have to have the mob hitting YOU to get the Vengance stacks, and you have to hit the mob back -with- those Vengance stacks running in order to keep making more threat. Worse still is when you're sharing tank duties with another tank -- and if you're the offtank, you spend the first chunk of time -not- getting Vengance while the DPS climb the threat tree above you. When you taunt, the MT has all the Vengance stacks and you don't, so -they- are going to pull off of you....sometimes when you really don't want them to. (cf. Argaloth, Chimaeron, Occ'ulthar....)
I run with a threat heavy build (agility, crit, and agi trinks) which is a little light on the health pool compared to most tanks, but with high agility also comes higher dodge, and as such I stay alive better than the stam stacker tanks. I'm presently running with 7% hit and 20 expertise, and that's enough to keep threat off of an entire 25 man group and steal threat from two of our MT folks without trying very hard.
Your mileage may vary; just relating stories from my own experience plus my housemate's.