We had a scenario where client runs a cluster of events, and folk sign up for these, and usually the registrants signs up for all events, but then they might invite mum to the Dinner, and brother John to the Talk, etc etc.
We wanted to achieve this on a single form with a single payment. We explored both CiviCart and Drupal Commerce but in the end concluded we could achieve this in a much lighter way with good old webforms.
The outcome is that up to 6 people can be registered for any combination of events, eg
To see the full gory details of the conditionals approach we took, please read the full blog on Fuzion's site.