Closing Air Intakes

[XLjedi]

Is there actually any gameplay or performance benefit to doing this? 

I'm inclined to just leave them open all the time.

[Tex_NL]

The theory behind closing the intakes is to reduce drag. In previous KSP versions you could squeeze a few extra m/s out of your SSTO plane by closing them. But currently, at the altitude intakes no longer serve a purpose on an SSTO plane, actually closing them does not make much difference.
Much easier to leave them open so you won't scratch your head why your engines won't light on the return trip.

[FancyMouse]

Just tried (in 1.3). Drag value (in aero action menu) shows there's absolutely no drag change upon opening/closing intakes.

Now I can probably get rid of the habit of adding closing intake to action group.