SR has a rule in which one can Banish an enemy mage's spirit to zero services, then use Summoning to become its master. So can any other mage in LOS. One can then bind the spirit.

Well, it isn't actually limited to an enemy mage's spirits. It could be a teamwork move, or a deal.

So do mages intentionally swap spirits? Can Shaman A (who knows Invoking) and Vooduin B make a deal in which each deliberately brings a spirit to 1 service and lets the other Banish, Summon, and Bind? Vouduin B ends up with a bound materializing Great Form Beast spirit, and Shaman A ends up with a bound possessing Task spirit (for the next time he needs boosted physical stats and (Force) ranks in Gymnastics). Of course, those spirits can't do Aid Sorcery, since each tradition matches a class of spell with a class of spirit, and the shaman doesn't have a class of spell that matches with Task spirits, nor does the vooduin have a class of spell matched with Beast spirits.

Since a Spirit of Man can have Innate Spell as optional power, this effectively lets mages trade access to each other's spells.

I'm imagining some Matrix forum which is kinda like EBay for spirits.

Especially useful for those max-CHA elven mages who can keep a posse of bound spirits, each specialized for various purposes. And hey, I'm currently playing one.

Interesting concept. I could see a magician with a (perceived) specific need reaching out to a peer from a different tradition.

Of course, you could just lend the other magician the aid of the spirit.

Loaning tasks has two major limits, first is duration. A spirit still counts again your bound or summoned limit, meaning that having it gone for a days or weeks or more is really not an option. This would allow a magician to barrow a task spirit for long enough to build basically anything for instance. Second magicians aren't on hand to loan services as readily as your spirits are to do them. Having a spirit of man on hand with possession, heal, and armor/combat sense just to pull people out of the fire is very powerful, but not in the time frame it takes to whistle up a contact.

How will the spirits feel about the banishing part? "No worries chum-pal, you be right back under control?" will sound like loss-loss to them. Even if I have to admit that it could end in Win-Win for the mages.