Well actually there is a lot of use in keeping it cached in Guest Caching. When vB Optimise caches a page, it doesn't store specific data for components - it stores the entire page output which includes whatever the page is displaying, then when returning this it avoids all the SQL queries normally used to present the data and PHP processing.
I'm not sure if vBET is doing the exact same thing, if it is I see your point but it'd be a case of either use the vBET entire page caching or vB Optimise. If vBET is only caching translations then pages will still be executing queries and processing PHP which vB Optimise avoids for guests.