Lawyers will have many opportunities to subtlety shift a discussion one way or another - in fact, they might not even realize they're doing it. You are right to be concerned about subtle and systematic tilt in their perspective, and I would definitely not write off your concerns as moot because of any code of ethics. Only in interview questions are there truth-tellers and liars; the rest of the world operates in shades of gray.
But you shouldn't worry about this one too much, although not for happy reasons. Any $25k your lawyer may invest in the deal will be a drop in the bucked compared to another, much more influential bias: a VC can do a lot more for a lawyer's business and reputation than a typical founder. Entrepreneurs may found multiple companies and have big networks, but VCs go through lawyer-hours like fast food fries. They simply hire or influence so much legal business (directly or through portfolio companies) that company counsel is already tilted as far towards the A as they can go.
So I say - let em in. Make it official. At least now they have to tell you when a conflict emerges.
