I think polyamory is beneficial to my marriage, and Hubby agrees. In our situation, I'm the only one who's polyamorous; Hubby has no interest in even having sex with others, let alone having other relationships. Some reasons we've found poly to be a good thing for us:
Hubby is a "loner." He doesn't enjoy interacting with other people. Going to a party, club, etc. is his idea of hell. Given the choice, he wouldn't leave the house at all, and in the past when we've tried to do activities as a couple or family, he's dug in his heels or has argued about where to go.
I'm very awkward socially, and have social anxiety, but I like being around people. Our local group on the dating site where we met has frequent "meet and greets" at bars or clubs, and I enjoy those because I know most of the people and because there's dancing. I love seeing new places, going for long drives, etc.
Hubby hates having long conversations. He can only stand to listen to someone for a few minutes at a stretch.
I tend to process things verbally, because if I try to think them through, the reality and rationality gets tangled in the morass of my depression and anxiety. Speaking aloud helps me get the "right" things separated from the incorrect perceptions. I also tend to talk a lot about things that excite me or make me really happy, like writing, music, etc.
Hubby is very, very conservative sexually--for himself. (He doesn't care what other people do sexually; he doesn't consider it his business.) He did all his experimentation and exploring as an older teen/young adult, and knows what he likes and doesn't like.
I'm... not so conservative. As a teen and young adult, because of abuse and because of some seriously warped ideas my mother gave me about sex (women shouldn't enjoy sex, but have no choice about it, for example), I didn't explore or experiment. I was too afraid to. In my first marriage, my then-husband actually said to me, "You aren't allowed to enjoy it, because that would make you a slut. But you'd damn well better pretend to enjoy it so I know I'm a man." And unfortunately, in the early years of us living together, Hubby reinforced that I shouldn't initiate sex, shouldn't "want" it, should just take what I could get.
During the time between my first marriage and meeting Hubby, I *was* able to explore and experiment sexually. For the first time, I was able to allow myself to want and enjoy sex, and to accept myself as a sexual being. Unfortunately, within the first year and a half of our relationship, Hubby pretty much undid the gains I'd made. I still didn't completely know what was or wasn't "okay" sexually, so I was basing things on his perceptions, because I hadn't formed my own fully and because I was afraid he would react similarly to my first husband if I tried to stand up for my right to have and enjoy sex.
That was what led to us opening our marriage two years ago. I finally got fed up and said, "Look, I get that you don't want to try the things I want to try, but it's wrong and unfair for you to expect me to define my sexuality and sexual interests by yours. Basically what you're saying is 'Hubby gets to decide KC's sex life, and KC has to deal with it', and that isn't okay with me anymore." Hubby's response, after he thought it over for a couple days, was to say that, while he wasn't comfortable with going "out of the box" (or out of the bedroom) sexually, he wanted me to be happy and comfortable, so he gave me his blessing to find other men to explore with.
So... the shorter version of the ridiculously long answer:
Polyamory benefits my marriage because Hubby's and my communication has improved. In the process of setting boundaries and agreements for the open marriage, we learned to say what we really meant, stand up for ourselves (me particularly; Hubby never really had a problem with that), and truly LISTEN to each other. We were on the verge of divorce at the end of 2012; by summer 2013, our marriage was stronger and happier than it had ever been.
Polyamory benefits my marriage because it gives me the opportunity to explore sexually with someone else. S2's completely open-minded and willing to try just about anything (once I get out of my own damn way and stop being afraid to tell him what I want to try).
Polyamory benefits my marriage because I have someone else who loves and supports me, and is willing to listen to me process things, vent, or share my excitement about things in my life, when Hubby isn't able or willing to listen.
Polyamory benefits my marriage because I have someone else who takes me to the parties, the clubs, and on long meandering road trips that may or may not have a destination. S2 is also going for walks with me, and will be taking me hiking and camping, because he thinks it will be good for my health. I'm not always comfortable doing those things by myself, so having someone else with me definitely helps.
And from Hubby's perspective, which he has stated to me more than once, polyamory benefits our marriage because Hubby's job is fairly dangerous and risky, and with someone else in my life, Hubby knows that if something happened to him, I would still be taken care of and loved.