Heh ... the possible benefits will vary, but in some cases ... bromance?
We've talked about NRE (New Relationship Energy), we should also talk about RRE (Renewed Relationship Energy). RRE is when your wife is "energized" by the new relationship in such a way that some of the NRE "spills over" into your relationship with your wife. So there's another possible benefit.
You mentioned how the other guy is a friend to you and treats you good. If he is involved in your life, he can be like an ally who's got your back. Another potential benefit.
I would hope that your wife would be okay with it if you decided you wanted to go out and seek someone new that you could date. If so, that would be another benefit.
An MFM V can be a tightly-knit group. Mine is. It gives me a feeling of security to know I have two, not just one, person who has dedicated themselves to my well-being. We have a lot of fun together. We enjoy life together.
You may say, "But since two men are sharing one woman, each man only gets 'half a woman,' mathematically speaking." Mind you she loves both of us 100%, but she can only allocate 50% of her time and energy to each of us.
Which sounds like a drawback, but I personally actually like it even better that way. I'm quite a introvert and greatly enjoy my "me time." I get to do more of the stuff I like to do, stuff I can basically only do by myself anyways. So, depending on how much like me you are, you could even think of that as a benefit.
Am I saying everything about polyamory is beneficial (especially to the spouse in your position)? No. Monogamy is a special type of relationship and you have to sacrifice it in order to engage in polyamory. Polyamory can be difficult in the first few years. It can take time for everyone to get sufficiently used to each other to start enjoying the benefits. There is hurt and heartache to be endured while you're all learning how to live in each other's lives. And like you said there's a risk to your marriage.
But you said it yourself: Your fears about the loss of your marriage are probably unfounded. Pure fear is something you can pass through, with courage and endurance. Like a storm, it floods you with rain and lightning but then, in due time, it passes you by and the Sun reappears.
So there are drawbacks, but they're not unsurmountable.
Oh, and there's one other benefit. The joy of knowing that by giving her your blessing, you're doing something for your wife that makes her tremendously happy. The happiness we shower onto others very often splashes back on ourselves. I'd consider that beneficial.