From the looks of it, Devs wanted to create and incentive to go 51 points into RS, which was pointless in beta 6. The issue that was created is that the intensive is overboard. Basically for every point you gain .5% damage reduction, 1% endurance and 5% on guard shield size. Frankly no other tree offers anything even remotely similar tanking gains per point. Most of the Tanking goodies are in first 8 points of Bard,Ranger, Blade. Since we only have 2 souls besides RS, you can only get 2 of them so you need 16 points to get the most of it. ( and you end up with 15 so you can...)

Ranger: 5% hp 6% damage reduction - 8 points Nothing past 8 points.
Bard - 10% endurance , 3% miss chance + armor anthem (in 5 mans) few situational 5 man things parst 9 points, nothing MT worthy, in 20 man raid all barb buff should be covered, better to have dps loose 10% dps then gimp MT for it.
Bladedancer - 10% dodge, some threat, 50% dodge cd - 8 points Past that you can have 15% dex and 6% parry. and more threat/ combo point generation. + interrupt (which might be required in some cases, but it seem that for MT is better to have ofspec with interrupt then to have it in main build.

So the only tree worth going into for a tank is bladedancer, However lets look at price you have to pay for 15% dex and 6% parry and some threat: 15% endurance, 7.5% damage reduction, 75% HP large Rift Guard(should not matter, since you absorb less now) , some smaller threat/utility talents Deffer death/scatter shadow combo (which by looks of it right now is 10-12 sec damage immunity, which is insane.. imho), Aoe taunt.

The way i see 15% dex + 6% parry ~ 7.5% damage reduction ( best case for for parry dex) Guranteed reduction >>> rng by a mile. But even if they are even that still leaves 15% endurance, aoe taunt and DD/SS combo that has nothing comparable.

Bottom line as it is stands right now, rogue tank are forced into 51 points in RS. which kill outright whole point of flexible specs.

Possible solutions:

This is not important part but i figure i d throw it out there.

1)Nerf top of rift stalker tree bonuses, while buffing basic abilities (something like 25% Rift Guard with 10% more from max points)
2) add few tanking talents to ranger and bard in 8-22 point range.
3) Something like soul link with pet to transfer 5% damage to pet would be amazing and unique style of RS/ranger tank.
4) some self buff motif in bard etc...

This would allow for 51 RS/ (bard/ranger/BD) or 40ish RS 22is (Bard/Ranger/DB) + 6-8 left over tree. This would give a ton of possibilities, make each rogue tank Unique.

End result being something like :

51 RS + w/e - bit more endurance, high riftguard, aoe taunt, extra tank cd with DD/SS combo (if that will not end up too OP)

RS/Ranger + w/e - Tank with a pet, where pet soaks up % of damage so it needs to be watched as well (fun, unique)

RS/BD + w/e keep as is.15% dex 6% parry some aoe threat and combo generation, interrupt (obviously a bit buffed compared to now due to basic abilities of rift Guard and Guardian phase buff, now viable),

RS/Bard + w/e have a unique self buff motif that needs to be kept up, making a true hybrid between rift stalker/combo point self buffs and motif mechanic from Bard. + providing situational extra anthem/fanfires if 5/10 mans mayebe even in 20 man.

That is 4 viable tanking builds with smaller variations on 3rd soul. Amazing possibilities, all this with 2-3 minor talent/abilities additions in Ranger/bard that in one way or another provide 0-5% end buff and 5-7% damage reduction, as well as small tweak to RS scaling with extra points. Don't think 5% damage reduction in mid bard or ranger trees would make either of them OP in pvp. As for PvE 5% damage reduction is nearly irrelevant as you not getting hit that much, especially since you would have to sac some damage/support points for it.

If i am missing something and there are some other viable builds besides 51 RS please let me know, right now it doesn't look this way.

That is my 2c