Depending on the pen, it is also smoother when writing, offering less resistance. Useful to sketch diagrams quickly with smooth lines or have a nice regular writing.
Con: I have been using Lamy fountain pens for a while, and Lamy owners can recognise each other's in the firm to the traces of black ink on their fingers.
Obviously, the plural of anecdote is not data, but I've been using a Lamy (Logo; fine nib) for years now, and I've yet to have more than the (very occasional) weak smudge of ink on the part of my pinky and ring fingers resting against the paper.
I'm a leftie, even. (Mostly using Parker Quink, which dries very fast - occasionally using Diamine inks, too.)
Across multiple models but with broad nibs. Usually it leaks into the cap when closed and then the part of the pen the closest to the nib is covered with ink.
I need to take a look at this. Are you sure that you're not gripping from the lip which locks and creates an air-tight seal? The grip would make it very uncomfortable though.
I have a medium Lamy which writes broad-ish (because of production tolerance + well smoothed nib due to long writing sessions) and wets its sealing lip, but even that one never stained my hand.
Which ink are you using? Is it extremely wet and eager to flow? Do you fly with the pen? Temperature around the pen?
I've been using only Lamy pens for the last couple of years, my kids too. We have about 10 of them, and we usually use the cheapest ones. The ink is also made by Lamy. And we have no such problems.
Con: I have been using Lamy fountain pens for a while, and Lamy owners can recognise each other's in the firm to the traces of black ink on their fingers.