After reading
"Monster Rancher Metropolis: Monster Rancher 4 Archive (PS2): Research on game mechanics: Phoenix Lifespan Tests" and Petit-Trot's comment, I was wondering if a test is needed to see if combining trained monsters at a high Bonding level is any different than 2 untrained monsters at a high Bonding level.

Say we have 4 Rippers, R1 and R2 were trained, Best Friends, and they the same maxxed out stat. R3 and R4 were rested their whole life, and also had a bonding level of Best Friends. Would there be a significantly higher stat for the trained-parents baby than the untrained-parents baby?

Or if we have R1 and R2, who weren't even Friends, and we have R3 and R4, who were Best Friends. Would R3 & R4's baby possibly have higher stats than the trained-parents baby?

I remember seeing a chart for how to find the stats of a combined baby monster, but it doesn't seem like the stat can get any higher than 1.5 a monster's original stat... wait now I'm confused. Can it? A baby having twice of a book-monster's stats?

There's probably a hole somewhere in that whole thought process... that's why now, rereading what I just wrote, it doesn't seem to make much sense...

Monster Rancher Metropolis: Monster Rancher 4 Archive (PS2): Combining/Breeding: Stat Caps on Combined Monsters

Second post explains combining in it's entirety. Bonding essentially plays a far larger role.

If a 2 untrained, and Best Friend monsters were combined, the baby would have the same stats as a baby from 2 trained, and Best Friend monsters? (if the stats -high to low- are still in the same order)

that's my conclusion from reading "Monster Rancher Metropolis: Monster Rancher 4 Archive (PS2): Combining/Breeding: Stat Caps on Combined Monsters "

but it just seems wrong, because then what would be the point of raising good parent monsters for a good baby?

It's not necessarily true in that sense. There is some room for improvement in pretty much all cases (especially when you fiddle with the order), but there are limits.

Besides, you have to train to get the best benefits from friendship. Being "Inseperable" makes a huge difference. "Best Friends" won't do a whole lot.