Here's a little further discussion from a blue post on the T13 drops. They at least acknowledge that RNG will, in so many words, screw some players.

http://us.battle.net/wow/en/forum/topic/3196392331#16

"Ultimately, we recognize that making these set pieces available only as raid drops leaves players much more subject to RNG drop rates/loot rolls, but we have a couple of plans to try and alleviate the sense of burden that stems from the randomness of token drops.

The way tier 13 pieces will be obtained in the 4.3 raid won't be unlike raids past, meaning players will still need to roll on multi-class set tokens. That said, we'll be tweaking the tier 13 10-player drop rates a little bit (for the better) relative to the 25-player version, given that these tier pieces can't be purchased from vendors for Valor Points.

In addition, due to tier pieces not being offered for Valor Points, another thing you'll see in patch 4.3 is a much wider array of desirable non-set loot (covering more slots than prior patches) available from the vendors, for those players who are unlucky with specific token drops, or who aren't interested in raiding -- although, once you get more details about the Raid Finder, we hope you'll find interest in having a go.

I felt this warranted its own response in a separate thread, but for those who missed it, here's my response to the original thread about tier 13 pieces being raid drops only:"

http://us.battle.net/wow/en/forum/to...33?page=23#456
Keep in mind we do intend to allow tier 13 set pieces to drop for those using the upcoming Raid Finder. Similar to the difference between normal and Heroic versions of tier sets, the Raid Finder set will have a lower item level than the normal and Heroic counterparts.

Set pieces from all three difficulty levels can still be mixed and matched for the 2- and 4-piece bonuses.