Actually no. Not really anyway, you're a bit off on your calculations. It would be 15% dodge and roughly 0.4% more on all your chances.
But I'll give you an idea based on my character's stats, so we can look at a fully configured situation.
Def R: 733
Dodge R: 652 (with Libram)
Parry R: 438
Agility: 343 (with SoE and Kings)
You have, unmodified by Diminishing Returns, 30% Block from HS, 5% dodge from Anticipation, 5% Parry from Deflection. You do *not* add Redoubt, since it procs only from direct hits. If you're block capped, you will never receive such a hit, and so Redoubt will never proc (saved stunned situations, but that's a different thing).
You also add your base chance to Dodge and what comes from base agility, both are class based and untouched by DRs. To make it simple, we'll assume the sum of these 2 is 5% (it's slightly more, but not much).
We get (mind it, *after* diminishing returns):
9.49% miss
31.36% dodge
17.14% parry
Which configures a 57.98% avoidance situation. Do my chance seem low? Check the
Diminishing Returns to discover yours!
I'm not kidding on this last point, my avoidance looks a LOT higher if you just look at the character sheet (totaling 10% more). But you must consider how DRs cut into your ratings.
Add to that the Block chance. It's 5% base block, plus 5.96 from defense, which (this is important) is *not* subjected to DRs, plus 30% from HS. Makes a 40.95% chance. Add to that my 43 block rating (T8 head) and you get a total of 43.58%.
The grand total becomes 101.56%. Yes, I'm below the Block cap. I have a serious bet that most people are and don't realize it until they get their hands on the DR formula. I'm not worried in my case because Mongoose alone is enough to push me over the Block cap, but I suggest everyone to check out their effective values post-DR, especially since the ratings changed dramatically after 3.2, Dodge and Agility on top of them.
To answer directly the OP question: Block Rating is a waste of item points. You can, and you will while you progress on TotC gear, get Block Capped without needing it. On this side, Defense is your best friends. Keeping some defense enchants (namely chest and shield, which are the best itempoints-wise) and some def/sta gems will help you stay at the cap once you've dropped your last points of Block Rating.
As you've seen in my case, I would be, changing T8 head, something around 3% under the block cap. However that gets greatly compensated by defense (which also affects Block) and TotC gear has LOADS of it. As last thing, if you find yourself at like, 101% avoidance versus a 102.4% cap... don't get scared. Crushing Blows aren't in the game anymore (for raids), and a direct hit is basically 2k extra damage when compared to a blocked it. So don't forsake the block cap, but don't cry over it either.