Are there any advantages to selecting the middle seat in an airplane?

• You have the moral advantage when fighting for the armrest. The person on the aisle or window has at least one uncontested armrest already.
• There's a slightly higher chance that the seat in front of you and/or behind you will be unoccupied. If in front, you don't have to worry about them putting their chair back. If behind, then you don't have to worry about them kicking you or about inconveniencing them by putting your chair back.

Yeah, not much advantage. I've never picked one unless it was to sit next to someone I know.

The middle seat(s) between the two aisles on a widebody plane shares one advantage of window seats: you won't have to get up to let someone else out, so if you're able to sleep, you have a better chance to do it undisturbed.

You can certainly argue for some, both objective and subjective.

• for exiting in an emergency, aside from sitting in the exit row, aisle seats > middle seats > window seats for ease to escape
• for exiting the plane, it's faster than a window seat in the same row
• if you want to be 'that guy', on airlines like RyanAir where it's sit where you want seating, taking a middle seat on a partially empty flight makes it more likely you'll get three seats to yourself as other travellers want to sit next to their friends/family and will sit elsewhere.
• less noise / chance of being bumped by a cart
• when the plane tilts a bit from side to side, you move less in the middle than the window (although again, aisle wins here)
• Sitting closer to the middle reduces air-travel sicness although clearly on a wide-body flight the middle set of seats would be even better. Still, better than a window for this.
• You get two armrests! Two is better than one, although this now leads to two potential arguments over the no-man's land that is the armrest. Some will argue that nobody should use them. And now you have no armrests that are ONLY yours.
• statistically greater chance that you'll have a spare seat on one or the other side of you than when you're in the window seat and have only that one chance.
• you have easier access to the bathroom than the window seat person.
• small one, but on the aisle seat I hate handing other passenger's dirty plates over me to the attendant, in case something spills. Middle seat has less chance of this happening, as they only have the window seat's stuff passing them.

The only advantage I can think of is the opportunity to have two neighbors to talk to. This can also be a disadvantage if you're particularly anti-social, or you end up with bad neighbors.

Which I guess goes to show why this question really is "Primarily Opinion Based."

If you are traveling alone it's hard to come up with a good justification. But many people travel in pairs. One will want the aisle or the window for whatever reasons people want those seats (I'm a hard core window person myself) and the other wants the middle because it's next to their travel companion.

Simple as that.