Entirely down to individual games, but the main reason I know a lot of people use them for is reviving their Greyfells. If you have many, you would burn through pandora's boxes reviving them on a monthly basis.

They also don't seem to come round very often in promos (or I'm not really paying attention), so it's more difficult to build up a good collection of them, hence why I think they get snapped up in the exchanges.

Also, the only way to get a horse back from a loan early, is to kill it on the loaned account. Then it returns to the owner to revive with a PB....

AND... value is relative... If you have a hundred Hera packs and no PB when you need one, then it is a fairly no-brainer trade, rather than spend 95 passes...

This is interesting. I don't own any Greyfells and neither I am I on a team so have no reason to loan a horse out! Thanks for this insight. Always things to learn on this game!!