I wouldn't say that there is a whole lot in common between the two. Having played rugby first then learned takedowns through BJJ I had to unlearn some of the habits that tackling in rugby creates. This mainly comes from the fact that the guy you are trying to tackle in rugby is trying to run through or around you so you don't really ever need to have a deep shot to get under their hips its more a question of timing and good body position. Also once you make a tackle, in rugby union at least, the guy isn't really concerned with staying up once they know they are going down rather protecting the ball and the rules make holding people down unnecessary so TDD is a secondary concern.
I wouldn't think that playing rugby specifically to get takedown practice would help your grappling training even if it was technically useful because you'd spend so much time training rugby just to get a small number of tackling drills in you'd be better of using those hours training grappling.