It may deter the less-than-fully-committed. (Whether the rest should be committed, I am not offering anything other than the opinion that they should be assessed.)
I wouldn't count on too much of a decline from what they would have had for reasonably good weather. Don't underestimate the depth of commitment from them - I've stood in 40 degree drizzle at 3AM, and it's not that rough if you really do care about what you're protesting.
With the media coverage it could have, it will underscore their perseverance - and make them more sympathetic to the like-minded or even waverers.
I would love to see them outnumbered 3 to 1 by riot-geared police without IDs, just so they get a taste of how that feels. Alas, that seems to still be restricted to DFHs.

...and maintain the populist imagery...

So they aren't even arguing that it is actually populist in nature?

I think my favorite piece in all of this is the argument that somehow these d.bags represent a majority of Americans. The majority of Americans want health care reform (with a public option), out of Iraq and support marriage equality. Compare that with the Bush policies that were so unpopular by the end of his reign that the candidates in his own party ran in opposition to them. And is it just me or does Obama seem a little right of the American populace?

A brief history lesson concerning the founding fathers and the constitution. This country was set up with the principles of majority rule and minority rights. And regardless of any Jefferson quotes you want to dig up, nowhere did they advocate violent revolution to institute minority rule because a small segment of the population is dissatisfied with the outcome of an election.

Or to phrase it more succinctly: scoreboard.