Like, "Bitcoin is anti-American" or "Bitcoin will be used by heroin dealers who will sell your daughters into prostitution"
If bitcoin is a means to circumvent the tax system, then we'd have to apply whatever tax regulation that results to all other forms of currency. Social Security is an illegal pyramid scheme because it wasn't sold to the American public that way.
Let's start applying the law equitably before we stomp on innovation.
That is, any "money" is both a medium of exchange and a store of value. It's OK to compete with the dollar as a medium of exchange but if you compete as a store of value, then you've wandered into some heavily guarded turf...
Bitcoins are probably most at risk from being classified as a type of security, which would put them under the scope of a large number of financial regulations. It wouldn't make them illegal, but it would make them more difficult to trade.
However, in general I'm optimistic about Bitcoin's chances. I certainly don't believe it is inevitable that they will be regulated out of existence.
One argument would be that bitcoin is an illegal pyramid scheme.
Another would be that bitcoin is means circumvent the tax system (all barter and local currency systems, for example, have to pay taxes).
I'm sure many other arguments could be found.