Fey RHD, with their low HP, poor BAB, and general unimpressiveness, aren't very useful on a martial character. The MM3's developers solved this issue like they solved most: with hit dice bloat.
That said, the thorn's six RHD do hide a number of interesting features. +2d6 sneak attack, +4/+6 to all martial stats, and Small size actually make for a rather decent rogue-type, and having a pixie's Sleep Arrows doesn't hurt either (why yes I like having on-demand save-or-loses). DR 5/cold iron isn't half bad either.
Compared to a level 7 rogue, a level 1 thorn rogue has 1d6 less sneak attack, 2 less BAB, less class features (Evasion being the big one), and none of whatever racial features the rogue might have. However, they have DR, superior physical stats (as well as charisma), and the aforementioned sleep arrows. Then again, the rogue could by now easily be two levels into a prestige class.
Given the excellent arguments both for +0 and -0, and the equal results in a subsequent vote, I think that either rating could fit them, to be honest. Look at the optimization level of your game when deciding whether it's worth playing, otherwise just subtract a hit die.