I am not a lawyer and am not giving you professional advice here. Merely discussing the topic.

I see what they are getting from you...but what are you getting from them?

You lose control...they get your expertise...you can be booted out at any time thereafter.

I don't see the upside.

If they want your expertise, they can pay you for it: hire you, license your product, etc.

If they don't want to work with you as a separate entity, that should tell you a lot about their true intentions and how badly they truly need you (or don't).

I personally would not get involved in an offer like the one you've described. You can still be friendly, offer to work together, be complimentary as you say, but I don't see any reason for you to give up control for a few shares in someone else's company, transfer your knowledge, and then be at risk of being kicked out because they don't need you any more. Plus, do you really want to be a minority shareholder and treated like an employee, rather than the owner?

I suppose you could have a legal agreement drafted but secrecy never lasts and your insights WILL be leaked sooner or later. I also doubt your position in the firm can be protected. Speak to an attorney as I am not an attorney.