That's some interesting thought you have there, for a crystal clear picture the raw differences in performance actions are listed below, not to mention you can see the impact on sites using Pro here: http://www.dragonbyte-tech.com/vboptimisesites.php
If you think Pro proves no advantage over Lite however that's your opinion, people who are satisfied with Lite can continue to use it all they like
Lite can cache:
- Styles
- Templates
- Datastore
Pro can cache (in addition to above):
- vB Cache Datasystem (CMS/Sidebars)
- Showgroups
- Forumhome WOL
- Guest Full Page Caching
- Forum Display Query Enhancement
- vBulletin Notices
Maybe they made the lite version too pro then, so at least be greatful they made a free lite version. Honestly tho, the pro version has many benefits over the lite. But if you are happy with what he lite version offers for free then great.