Easier to show rather than tell... I'm on Ravelry, so if you are a member take a look at some of my projects. There are over 100, a few are top down but most aren't. I'm not advocating that no one knits top down - its quick, fun, and painless due to no seams! But I've looked at a lot of top down baby patterns, because baby garments are what I knit, and seen no where near the variety of styles and little features that I find in "pieced" garments. Also, I never seem to like the shape of the hoods in top downs, and the armholes sometimes strike me as a little skimpy looking and the body also sometimes strikes me as a bit straight and narrow and up and downish. I would love to find the sort of styles I choose to knit in patterns that are written top down and when I do, I'll be knitting them!
I do know its possible to convert a garment and do it in the round, top down. But I don't feel inclined to do so, especially too when I don't have faith in my finished product looking as nice as, or exactly like, the original pieced pattern. So I guess I'm not as clever in that regard as a lot of other knitters.
I feel that its not always possible to achieve the exact same effect, when converting to top down, as the original pattern (speaking baby knits here) when the pieced pattern is a little unusual.