>>542373483
You do get everything even if you borrow her. What he is talking about is the "event power" that depends on the LB status. This event power affects how much stats, energy recover etc. you get from her dating events and random events.

Top row has SSR 0LB and SSR MLB
Bottom row has R 0LB and R MLB

This is just the first event where the difference isn't too big, but you can see that SSR MLB gives 13 stamina instead of 12 stamina that the R version provides. It also has "energy recovery ammount up" which means higher LB gives a little bit more energy if the event has energy recovery. MLB version of course has couple of things going for it aswell such as the unique effect which R version does not. SSR Riko unique skill "Rushing Gale" is also not that good so missing out on that isn't really important.

Unique Effect
Decreases failure rate when training together (10%)
Decreases energy consumption when training together (5%)

The reason why people say MLB R version is enough, because the difference between the stats and effects is rather low so MLB R more or less compares to 1LB of SSR version and can serve you enough to not having to lock your borrow slot to Riko. You can also potentially save pulls as you aim for MLB R version, however do take note that this isn't like just couple of ten pulls even if it's R. You can potentially end up using a lot more depending on luck of course.