Probably had the most challenging battles a player could take on at the time in the series so if you wanted tricky, arguably rigged against you matches to try and solve with either your own pokemon you've raised up or the rentals it had it's relevance. Losing the ability to use your own pokemon does make things a bit less fun to be honest.
It's that or you spent a lot of hours away as a kid in the Mini Game Corner.
If you want a challenge, generate a random team of rentals with the Random Pokemon Generator
https://randompokemon.com/ or a Random Number Generator and pick the pokemon associated with the number and see how far you can go.
Do better than Keizaron dying to a 1hp Magikarp
View: https://youtu.be/7GHdAtbPYvo?t=1376