No, it can't.
Look, I'm all for free speech and the legalization of marijuana (even though I don't use it). But it's a stretch to equate smoking weed with expression. In fact, doing so only hurts the cause.
The case for legalization lies in lightening the legal and financial burden on the U.S. justice system caused by possession arrests. It also lies in reversing the politicized criminalization of a substance that does less harm than many legal counterparts, and actually has legitimate medical applications. If and when marijuana is decriminalized nationwide, it will be because of those or similar practical factors.
The case doesn't lie in the freedom to smoke pot anywhere you want, at any time. That's not going to happen even if it's legalized. It's still going to be restricted by age and in most public places as cigarettes are. Employers will most likely still screen for THC. Even if pot use does rise after the fact, it'll probably mostly happen behind closed doors rather than turn the nation into a stoner comedy.
Actually, I'm not sure legalization would change the culture much, because 1) anyone who wants to smoke pot already knows how to get it and 2) I can think of only one time I've heard someone say they wanted to try pot, but would wait until it's legal to try it. And she didn't live long enough to see the day.
As much as I love free speech, I also want to protect its definition. It's about expression in all of its wonderful forms, but not about every single action a human undertakes. Plenty of those actions are legal, justifiably illegal or should be legal, but they aren't First Amendment issues. What does count as legit speech must be governed by two principles — 1) equality of the right and 2) it must not infringe upon the rights of others or incite danger.
Though the Supreme Court disagrees, I think equating money to free speech violates the first principle. Tying money to speech means that some people have more free speech than others. The second principle is evident in the notion that you can't yell "FIRE!" in a crowded theater. Smoking as speech violates this as well, because First Amendment protection shouldn't apply to blowing smoke in another's face. While I doubt the Supreme Court will ever consider drug use as just another form of expression, I shudder over the precedent that such a decision would set. If you can't curtail peaceful speech in a public setting (and you shouldn't), how would you be able to govern secondhand smoke? Or worse?
There are far better ways to argue for legalization that aren't as fraught with peril, or are as ridiculous.