With the LFG/LFD system in place allowing folks to start queuing at level 15 the current design of having Guardian Phase available at level 18 makes tanking for rogues a bit of a trial. More to the point warriors get some fairly serious threat boosters around level 6 (Warlord), 7 (2/2 Aggressive Guardian), or even 11 (Sinister Intent) points in. Clerics get Mien of Leadership at level 12 as well which is plenty of time for them to get ready to start tanking 5-mans at 15.

So why do rogues have to effectively wait until level 18? Swap the root abilities around so that even if Riftstalker tanks are still behind the curve they aren't as far behind it as they are now.