I know emu junkies might get addicted to scanlines, but a PC never had sufficently low enough res monitors for the effect of scanlines. So unless someone really wants to add it, there's really not much call for it.

CGA screens had scanlines in every modes.
My Ega screen had scanlines in all 320*200 and 640*200 modes, but not in text mode and 640*350.

Never liked scanlines personally. Gives off too much of a TV effect.

emu junkies might get addicted to scanlines

They are also addicted to sai2x or advmame2x.
But to me those blitter look awfull and suck the cpu.
Im sorry, but when i played on my cga pc back in 1990, it really didnt look like the advmame blitter, but they added it...
Closest ive seen is mix of 50% scanlines with bilinear filtering.
And with the help of d3d, it doesnt eat more cpu. (but i understand d3d support wont be implemented in dosbox though, that why simple scanlines would do the job, to me).