No. It doesn't make you a liberal. However, voting for one that you absolutely know won't pass could well mean you're nothing but a hypocrite trying to gain political cover. Especially when the context is you trying to gain political cover because you just voted to borrow and spend another couple of TRILLION dollars. Which could well mean you're a liberal, eh?
The reason to have a vote on the BBA even if it will not pass is to demonstrate the actual position of the Dems to voters who will then elect conservatives who will support and pass it.
Maybe it won't pass the first time or the second but we need to have the conversation out there. We can make it an issue in 2012 we can make the RATs wear it.
While we are at it we can do a cap bill too and that doesn't need a 2/3 majority. Just doing CCB in stages.