I recommend it for both reasons.
The question I have now, are you recommending it as a 'motivational' tool or a legally binding agreement to protect myself?
Imagine a common scenario: One person on the project leaves.
On the legal side, without a collaboration agreement in place the entire project becomes legally tainted. The person who left is still a copyright holder with full rights to everything they did; their work and everything derived from their work is still owned by them. Except now they are gone. That is a legal poison-pill for the project. One person leaves and the entire project becomes a legal black hole. You must get the rights from that person if you intend to do anything with the project.
To prevent the project from becoming legally tainted you need to go back to every person who has ever contributed to the project and get them to sign a collaboration agreement, or if they won't, at least a rights assignment. Without that in place the project is legally dead; without that I agree with Tom that you should run for the nearest exit.
That needs to be done quickly, the sooner the better.
Failing that the project becomes a legal nightmare that you can never sell, license, or even give away. The only legal option for such projects is to abandon them to the wreckage heap of failed projects.
I hate to sound negative about it, but a project without an agreement is fatally flawed from a legal standpoint.
On the management side any cohesive structure will help the team any time there is a morale hit. One person leaving can be a major morale blow and often will results in a negative ripple effect. Having an agreement will help rebound the damage faster. You can re-assure everyone that even though the person is gone the project is able to continue.
As for motivation, people change. That is a basic simple part of human nature.
When a person is no longer motivated to work on the project your best option is to make a clean and swift break. Taking to long hurts everybody, both the person who lost motivation and those who remain. Chances are good that if ties were severed with a few key people earlier-on, the project's morale would be better overall. Now you still have to sever ties with those people but additional damage has been done. Left for too long a few unmotivated people can cripple any project.
Even at big studios where people are generally happy there is still employee turnover. People lose interest and decide they would rather become teachers or botanists or screenwriters rather than make games. Presenting the fact that the person is following their passion in a positive manner is very helpful.
There should always be a private side of the break and a public side of the break. The private side is often very difficult and personal. It is unique to every individual.
The public side that the team sees, if you can do the break on positive terms it helps mitigate the ripple effect.
"We love what Bob has done for the team, but now his life is taking a different direction. Bob, if you ever decide to come back we'd love to have you. We're sad to see you go but glad to see you following your dreams. We all want to thank you for being a part of the project."
"We're sad to see Bob go, but we're glad he gets to follow his new passion of stunt car racing! Let's give him a cheer; pass around the drinks!"
Without the collaboration agreement in place, when anybody leaves the project the entire project is instantly and automatically doomed. No amount of positive spin can change the plain and obvious fact that the project is doomed.
So get that collaboration agreement in place post-haste.
: Clarity, but more words.