I've posted a lot in my blog about the various issues and insecurities that come with my secondary relationship with a woman who has a primary. I'm happy with our roles in each other's lives... weekly-ish visits, acknowledgment of our relationship by our friends and parents if not by everyone (she's out to some people at work but she would probably never come out to certain people or to her grandma, for instance), a commitment to keep the lines of communication open, an intention to reestablish sex once feasible (her recent pregnancy and childbirth have put that on hold). I know that she values me, lets me in and trusts me more than anyone but her husband, and goes out of her way for me. Yet I do struggle at times, I have visions of a closer life together and I sometimes feel down when I realize that'll probably never happen.
A few things make it easier for me. One, I'm good friends (and then some) with her partner, which makes him an ally rather than a competitor. Two, I deeply respect their relationship... they've been together for 12+ years and married for 6, and have the vast majority of their issues well worked out. And three, there are really no hard and fast rules between us. Sure, my gf would probably end things with me if her husband absolutely insisted, but there's no explicit veto power that ensures that. It's pretty unlikely that we'd ever be able to become co-primary but there's nothing saying we couldn't. It lets the relationship feel organic and open-ended to me.
I don't know if I could deal with an unconditional veto power held by a metamour I didn't know well and trust. For a casual fling sure, but a serious relationship (and yea, secondary relationships can be very serious) where I'm investing a lot of time and energy and making myself really vulnerable? That might well be too scary to handle.