You lucky Bas--fisherman!!! I used to use a glass universal, but you are correct, dust was more of a nuisance than with regular carriers, but very convenient. You will just have to be tidier in the future if you use a glass carrier. Flatness is better on larger negatives, but when you stop the lens down it doesn't really matter. Bottom line for me would be cost of purchase and shipping, I'm a cheap-skate.
And while it is not as much of a factor with MF as it is with LF, its still worthwhile learning how to deal with the dust to take advantage of the flatness. Your prints will love you for it.
You got it. Dust and possibly Newton rings if you are unlucky.
If you have a condenser enlarger it will drive you crazy. If you do big magnification enlargements, it will be essential. As you magnification increases, your depth of field at the negative gets smaller.
The glass carrier really helps keep everything in focus, especially the frames at the end of the film strips, which are prone to curl more. I use the glass carrier all the time with my Saunders/LPL 670 VCCE enlarger. Make sure to get one with (or order) the optional Anti-Newton Ring glass for the top glass.