I've said it before, and I'll say it again.
Ability to use HM's should have absolutly no effect on teiring in B/W. The only times you are required, by plot, to use a HM is Cut, right at the start, which you could just catch a Choroneko to use, it's not like it's any use anyway. The other time is Surf, to get to one of the Sages.
I'm unsure if Strength is needed for the Sage @ Dream Lot, but I don't think so.
I do find it hilarious that the normal types on Route 1 and Route 2 completely outclass the cat even as a dark-type attacker.
By the time Purrloin gets Persuit [Lv 15, BTW], the normal-types have Bite, and Patrat is 1 level from Crunch.
If, for some bizzare reason you're still using Liepard at Lv 31, you get Assurance... which is STILL outclassed by Patrat, and the dog outclasses it too bith Bite, due to far better attacking stats.
You know what? A pokemon which is nigh-impossible to train even against the other derps [STAB Base 50 Tackle v No STAB Base 40 Scratch :/], and OUTCLASSED, even when considering his own STAB, by them, dosen't even deserve being in the lowest teir that exists.
Purrloin is COMPLETELY useless, to a degree far beyond anything else in the game. It deserves it's own teir, frankly... or at least, being at the bottom of the bottom, not the top of the bottom. Name that teir whatever. Purrloin's usefulness is superior to... a Feebas or Magikarp holding an Everstone... and maybe Sunkern, who decided to stay as Sunkern.
It's outclassed from the word go. It remains outclassed for the entire rest of the game. Not even slightly outclassed, but TOTALLY.