I have seen these parades in my hometown as well.
I actually find them a bit silly.
People are waving signs, listening to reggae music and wearing colorful clothes.
The silly part is that they only strengthen the bad public opinion about weed, instead of doing any good.

The public opinion is that weed turns people into lazy losers who waste everyone's time and lack any sort of purpose.
There are also people who believe that weed ruins the health, but most of them don't think so. Neither do I.
But I do believe that weed makes people lazy and empty headed. I believe it because I have seen it in my friends.

So what do people achieve by making such a parade?
They show to everyone that they have nothing more important to do than waving signs about how they get high. Waste of time.

To me it seems a bit like the PRIDE parades of gay people. They say they want respect, but walk on the public streets with dildos in each hand. My friend who is gay is ashamed of the parade's existence.

In a similar way, people who actually need weed may be ashamed of the ones who represent them on the weed parades.