I imagine this feat could be very useful in a campaign where players do not have carte blanche to buy items that suit them, either because they are low-magic or other reasons. So if you have your +1 longsword, and your weapon focus/etc. in longswords, but then you loot a juicy +2 flaming burst scimitar, this feat'd allow you to apply one of your Longsword feats to it.
That's actually the juicy part of it - you can skip prerequisites with this feat, so to speak. You could take Martial Versatility [Weapon Specialization] and get your +2 damage without needing MV [Weapon Focus]. You wouldn't get the +1 to hit, of course.
It wouldn't be something I'd take for that reason in PFS, because PFS allows one to buy items that suit them, but I've often heard of private games with more restrictive item availability; this provides an option for players in those games. It's always a bit disappointing to realize that the +4 scimitar is actually worse than your +2 longsword.
