Trap in the Owen Defence: Matinovsky Gambit


The foundation of that trap is just a tactical trick used in the Grob Attack by White. Owen's Defense is sort of a reversed Grob Attack.
http://www.chess.com/forum/view/chess-openings/grobs-attack-white-wins-rook

3...Nf6! should be played.
3...f5? is of course a big mistake.
6...Bg7 is Black's only move, when white has a few good moves. Best is the mildly surprising 7.Qf5!

I recently saw this in a video and I don't understand why anyone should play f5. Is there a plausible reason for black to play f5? I tried to create this situation but not a single person in the last 10 owens def games play f5.