As someone who buys a lot of charity bundles - I am fine with some of the entries being beta versions, or even limited versions or demos for a paid version. There's a ton of games on itch.io; I am never going to browse through them all. Someone putting their demo version in this bundle tells me that they support causes I care about (or at least, they don't object to causes I care about), and gives me a chance to look at them in a context other than "there are 35,000 games with this tag on itch.io."
That said, as an advertising ploy, it seems weak. I would have to really love the demo to consider buying the game because... I just got 500 other games to look at. The demo is going to have to really stand out for me to remember it the next time I'm looking at standalone games to buy.
...I love seeing the variety of games people are making, even if some of them aren't to my taste. If it's not something that fits my interests, I don't mind that I only got the demo version.