Personally the only reason having a license has helped me is through NARRPR which is a Nar Resource for Realtors that allows you to view statistics about any home in the USA.

If you feel the other agent is lying, that is a violation of the NAR code of ethics, so if you have good reason to believe they are lying you can file an ethics violation against them.If I am in a position as a buyers agent where I am escalating...after we are under contract I always request a copy of the 2nd best offer, or the 3 pertinent pages that make up the offer, the pages with the price, seller subsidy and other buyers agent name, so I can then call and confirm they made the other offer.

Having a license and being a part if NAR ( National Association of Realtors) it opens you up to getting referral fees from other states.   

And it seems that such regulations will be on the increase, as state lawmakers tend to copy what others have done, and realtors have strong lobbying with NAR, and quite frankly there is the bad behavior from those wholesalers who have wronged a seller that reaches the attention of the regulators and they then get to work on "correcting" that. 

I'd say go for it, just don't bother being an NAR member because that's the most expensive fee tbh...

Are there any negatives, other than cost, to joining NAR and local associations?

And signing a leasing agreement with you while you have an active agreement with another Realtor would be a violation of the NAR code of ethics. 

@Michael V Akbar my MLS charges $43 a month but I also have to be a member of the board and a member of the state association which also requires me to be a member of NAR.

NAR is changing this in the spring so you will be able to join the MLS of your choice regardless of where your broker is a member.

NAR has this thing called "compliance" when we agents help our clients rent, buy or sell.