Sure every pitcher is capable of being smacked around for a game or two. The problem with Liriano is threefold. First, he is capable of going into a funk where it isn't 1 or 2 games but rather 5 or 6 games where he pitches at a 7+ ERA. Just a few months ago he had 6 straight starts where he gave up 5ER or more! And it wasn't a fluke, he's had bad stretches in 3 of the last 4 years.
Second, Liriano has only pitched well in 1 month this year. Yeah that was a great month but even July he put up a 5.34 ERA with a 1.42 WHIP. Not exactly setting the world on fire right now. Even if you assume that his last start was a BLIP he still only has a 3.46 ERA and a 1.230 WHIP. While those numbers are decent they aren't elite by any means.
Third is a problem of perception. If you go out and get a guy like Greinke, Josh Johnson, Ryan Dempster or Garza your fans will be excited (obviously there is a difference between Greinke and the others here but there is a difference between Liriano and the rest as well). They will come to the game, spend money buying your overpriced hotdogs and beer and be excited to watch that new addition to the squad. That pitcher that is going to put them over the top. Now if one of those guys struggles down the stretch and you don't make the playoffs or bowout early your GM can still point to the fact that they went out and gave it their best shot. They went out and got one of the best and biggest names available. The talk show hosts and newspaper columnists can bitch but in the end they have to agree that each of those pitchers had the stats at the deadline to look like a good signing. On the other hand if Liriano comes out and goes 1-4 with a 5.80 ERA you have nothing to fall back on with your fans. He isn't one of the best pitchers and hasn't been for a few years. As a GM you're going to get destroyed by the local media.
So while Liriano's "stuff" might be up there with those top names bandied about all of the "peripherals" scream NOOOOOO!!!!!