It really depends on your agreement with the team leader and the team leader's relationship with the client.If the client is a frequent repeat buyer/seller investor I would say you are correct in your assumption, but agency agreements are generally transactional with a specific time frame start to end.  

LED is now mainstream and has pretty much become the standard.

My question is: will they only give their best if it's an exclusive agency?

I would call other agencies and check what it typical for your area.  

As for "mainstream" it's all about funding, marketing and "scalability"; Airbnb has all three.

Canada) and the credit agency you use, it may be possible to pull their credit history (even without an SSN).Failing that, is someone at the company willing to serve as guarantor for the applicant?

There are occasions where someone was looking for a product that would not be financially viable for me to peruse so I have declined to get into agency with them. 

I would suggest talking with some smaller mom and pop agencies.

Still even though it seems to be ignored by many different agencies I agree that eventually those that abuse the system will find that their actions have consequences.

Please be a bit more descriptive about what services you are looking to pay for.Disclosure, I am an agent but my business is not real estate agency.