Give me strength....!
 

Other than structural strength, is there any other reason why on most GA and light twins the ailerons and flaps are corrugated?

Can't think of one... but maybe someone else knows different?

Rigidity

Reducing spanwise flow

Because they can - the boundary layer is likely to be thick enough back there that the drag penalty is trivial.

I understand how the ridge type corrugations would help reduce spanwise flow but are the smooth indentation type as effective?

I imagine that spanwise flow is also less of an issue for light GA type wings than for high wing-loading swept aerofoils?

Corrugations add chordwise stiffness/strength/rigidity (as already mentioned) at a lower cost than the alternatives: adding a separate structure of stringers or ribs, with rivets to attach a smooth skin; or bonded honeycomb materials.

Cost in both money (materials, assembly labor) and/or weight.

At some point, as aircraft get heavier/faster/more expensive overall - the minor savings of corrugated construction become insignificant compared to other costs in drag, maintenance, total strength. Which is why they are mostly used in small, relatively slow GA planes.

As a side note - SPANWISE corrugations are being explored as a means of controlling/inducing vortices over the wing for aerodynamic reasons (analogies: dimples on a golf ball; stitching on baseballs). I wouldn't expect to see them on your local Airbus/Boeing any time soon - but you just never know....:8

Light weight is an important piece of the equation. The outer wing panels of the Swift GC-1A/B light two-seat caught my attention many years ago; had my arms been long enough, I could have picked up the pair of them and run off. Only four ribs (located to carry the flap and aileron hinges), and corrugated skins - very impressive strength/weight design!

Dimples laid over the A340 fuselage for a long term Airbus trial for none other than researching a fuel consumption improvement...

@ Ghengis the Engineer

Might this also help with flutter as it gets more rigid it has less tendency to bend?