I think it's difficult to generalize in this manner.

Since you've mentioned kAdhal Oviyum, my mind automatically recalled sindhu bhairavi (where some songs had to be lip-sync'ed).

'pAdaRiyEn-padippaRiyEn' had a brief interlude, appropriate for the stage, seemingly supporting your theory. Also, 'poomAlai vAngi vandhAn' has lengthy interludes as there's story being told there without lip sync, again supporting your theory.

'nAnoru sindhu', however, breaks this theory into pieces - both lip sync and lengthy, sweet interludes.

Bottomline - the length (& quality) of the interlude is situation demanded and director's story telling methods play the key role here. lip-sync or otherwise is unrelated to interludes, IMO.

(Another classic case is 'poongAtRu thirumbumA' of mudhal mariyAdhai', lip-sync plus lengthy / good quality interludes)