He IS a HUGE liability on obvious passing downs. He should not be in the game in situations like the 4th down TD to Moss. However, the O-line is THE main problem with this team. After the Skins scored that TD the Cards O still had a chance to put the game away and run the clock down. Instead, mainly due to poor O-line play, they went nowhere and gave the Skins the chance to put them away.
Watch other teams closely (not just highlights) and compare their O-line to the Cards line. You will see a glaring difference. I personally would take almost any other O-line in the NFL over the Cards line.
The interior line of Sendlein, Colledge, and Lutui is OK and should be kept. Next season however, the Cards should cut ties with "stand around" Levi Brown (the worst LT in the NFL) and move Keith back to second string. They should draft a high round tackle to play the right side and throw any amount of money it takes to bring in a capable LT.
With a good line the Cards should be an easy play-off team. As it stands with this O-line though, they are a heartbreaking 9-7 at best team. Sure, that might be enough to win the NFC west but any sustained success will elude them until the O-line is fixed.