Having moved most of my flying to other carriers, MM FO status has some benefits:

• Preferred seats at booking, including exit rows. Even when I was PM/DM, I'd regularly decline C+ "upgrades" in favor of exit rows.
• Zone 1 boarding still has enough overhead space available that you're not going to have your bag checked.
• Waived first bag fee.
• I actually got an F upgrade at booking when I purchased a same day ticket a few weeks ago. Granted, it was a random point to point route on an RJ, and if I had stayed in Y I would have had a row to myself, but still...

That said, I now have first tier status in all the alliances, so it's not much of a tie-breaker for me anymore.