It's exactly how lucid said as these are 3rd party plugins you are talking about here and not made by Scirra, so there could be various problems with that.
I like having plugins as a optional download as i wouldn't use some of them and they would keep making the main download larger so to be honest i wouldn't want all 3rd party plugins to be bundled.
The only problem really is that currently it's annoying to find them as people post them in the c2 general forum rather than a dedicated area. I suggested already to have a sub forum for these like there is with classic so they are easy to find.
There was a reply from the devs saying there is a plugin exchange section for the website planned but to be honest i would just prefer a forum section so it's easy to make bug reports and give feedback.