It would be a neat feature if the OSSC allowed for two input lines to be averaged for an output line. This could help give a different appearance to scanlines, as well as soften the bobbed fields with interlaced input and potentially reduce temporary image retention.
The reason for doing this in combination with scanlines in Line2X mode would be to average the pixel values where the black lines are superimposed. This would create a picture with softer looking scanlines, and in combination with hybrid scanlines simulate bloom.
As for reducing temporary image retention with interlaced input, the idea is that if the upscaled fields are blurred vertically, the contrast in flickering will be reduced somewhat.