People have already mentioned a lot of the main points, but with vintage pens you can also get some very good flex nib writers that can flex about the same or more than dip pen nibs. They're also easier to maintain in comparison to a frankenstein'd pen (ie; Jinhao x750 w/ Zebra Comic G shoved in) because you don't have to replace the nib (dip pen nibs rust over time) nor does it require quite as much maintenance to get maximum usage (usually something like the aforementioned Jinhao requires a cleaning after every usage in order to insure that the nib doesn't rust from the ink so quickly and hopefully get the most you can out of it before it does and has to be replaced).

Modern day manufacturers don't make flex fountain pens anymore because of the cost and lack of demand. We do have the Pilot/Namiki Falcon and its steel body version and it barely flexes at all. To get the level of flex close to a dip pen nib on a Falcon it has to be specially ground by a nibmeister in order to achieve that level of flex. Pretty much most modern "flex" pens are like that. So getting a vintage one would appeal to people. While some might need restoration, I think the overall quality you get out of it is a lot better. The quality of most vintage pens in general are better compared to now.