A fountain pen can be a perfectly normal highlighter

[morganp]

A broad nib and a bright ink could work well for underlining or marking notes, without needing a separate highlighter. The main thing I'd watch is whether the ink smears when you turn the page or write over it later. It probably depends more on the paper and ink combination than the price of the pen.

[penloop]

A broad nib and bright ink can make a bold stripe, but it may not have the see-through look of a regular highlighter. I'd check that the text underneath is still easy to read before marking up a whole page.

[inkstained]

The ink underneath is worth checking too: a wet line can disturb water-soluble writing if it crosses it. Try it over the actual text first, or underline beside the words.