Well, I prefer the black primer to the grey primer. And then there is always red primer. Black primer reminds me of the Ducati Monster "Dark". Imagine spending a ton of money to buy a brand new bike that was painted in primer? Especially a nice Ducati that should be red after all.
Oh, by the way, I've read that primer can attract moisture and facilitate rust, so it is not adviseable to leave rattle-can primer exposed to the elements for too long. I don't know all the details about that, but it might be better to use BBQ grill paint or even matte black finish coat paint.
That being said, I have some formerly chrome-finished plastic bits on my GL1200 painted in black sandable primer, and I think it looks great.