If Purrloin/Liepard are in their own tier, Raticate has a lower BST than Liepard. On top of that, Liepard has an alright SpA stat, so if you really wanted to give it some special moves, it would be fine. The only thing that really holds Liepard back is that it doesn't learn a lot of dark type moves and it's frail. That was really the reasoning behind my tier placement. In the long run, yes, Raticate is really not that good. It comes at a point where you should already have a normal type if you choose to use one (Herdier or Watchog), and it really is entirely too weak to stand up to two or more attacks. Saying "it can SD" or "it's good with Guts/Flame Orb" doesnt mean anything when it dies so easily.
It's worth noting that Liepard is slightly faster/stronger than Raticate, but Raticate has a better STAB move (Return vs. Night Slash) and has things like priority, Guts, etc. It is possible to use SD Raticate to some effect, sure it won't be as good as say, Minccino killing everything with Tail Slap, but it is a plus for Raticate.
Also 64/50/50 bulk isn't anything better than 55/60/70 bulk and Raticate can actually do stuff like SD/Guts, which Liepard lacks access to. In the end, the only thing that Liepard really gets is availability (seriously if it can do anything between it and Rattata this would be something) and a slightly higher speed stat, which isn't very significant for in-game purposes.
Having priority attacks is also a plus for Raticate as it allows it to do some decent damage while outrunning, and you also need to note that Watchog and Herdier lack priority.