If rentals exist to encourage players to purchase vehicles I believe they should always arrive with some kind of default recommended equipment to ensure they never get played empty. Nobody should be playing high tier tanks without three slots filled. Unfortunately since 1.8M credits is a large amount of credits not everybody wants to spend that much on a rental that'll go away shortly. If you have enough time, you can definitely earn your credits back and some more but not everyone has the time during the rental periods to do so, and they end up playing them for a few battles without any equipment. That's why rentals should come with something by default. For obvious reasons free equipment shouldn't be able to be demounted and kept but we should still be able to override default configurations by mounting our own equipment.
People will like vehicles more if they perform well during rentals, and ensuring nobody ever has to play one without equipment means more people may decide to purchase one after the rental. If equipment costs serve as a way to counter additional credits players earn during a rental there's an easy solution to that. Just disable bonus credits for rentals or hold their bonus credits in reserve for a short period after a rental ends and include them as part of the sale when buying it.
Bonus credits with rentals are great and I'd like to keep them on top of having them come with basic equipment but I think never having to invest credits on equipment for rentals would be much better, especially for newer players who don't have tons of credits saved up.
Source: https://www.reddit.com/r/WorldofTanks/comments/tv7k4c/three_empty_slots_is_just_a_bad_idea_rental/
