iDVD and Toast take very different approaches to serve users' desires in making video DVDs. iDVD has wonderful motion menus with sound while Toast has basic still, silent menus. iDVD is very limited in the video formats it can use for the source while Toast accepts a broad variety of video formats. iDVD only authors with uncompressed PCM audio while Toast can author with compressed Dolby Digital AC-3 audio which can leave more space for a higher video bit rate. Toast also has custom encoding settings whereas iDVD doesn't let you modify its settings.
What this means is Toast can't compare to iDVD and iDVD can't compare to Toast depending on what the user wants to have in a video DVD. Of course Toast does a lot more than create video DVDs.
If iDVD is meeting your needs then I'd say you don't need Toast for that purpose, but may want it anyway for other reasons. If you are finding iDVD too limited or too much work then I recommend Toast (or Popcorn 3) to give you more options.
As a disclaimer, I very rarely use iDVD because I usually don't have need for its menus, I prefer AC-3 audio on my DVDs and the source for many of my projects are existing MPEG 2 videos that would need to be re-encoded for use in iDVD. So I'm biased toward Toast. It doesn't mean I think Toast is "better"; just better for my needs.