I don't see, or should I say, I'm not particularly interested in relationships where the terms "primary" or "secondary" refer to a limitation of emotional availability for those relationships outside the primary relationship. It might end up that you don't share much of an emotional connection with a partner, but it's very important that I have the potential to have more than one person that I love romantically without restriction. I do believe that these terms refer to practical entanglement and joint responsibilities such as children, homes, financial obligations, family commitments and the like. I also believe that whilst one should avoid "couples privileges", it is absolutely likely that there will be privileges that a new partner has not yet earned. The key is that the potential should be there for those privileges to be earned over time.
In terms of veto powers, I would not be comfortable to enter a relationship with someone who had a partner who could end our relationship at any time they felt like. I am all for people having the space to approach their partner with an issue, or even say that they don't feel they are unable to continue in the relationship if x continues, that is simply expressing that your needs may have become incompatible, but as for being able to end their partners other relationships whenever they feel it necessary without reason or explanation is not ethical non monogamy in my eyes. You could perhaps make it ethical by stating to anyone who gets involved that they can be discarded in that fashion, but you must make it clear that quite possibly, it will be through no action of their doing and there won't be a compromise.
There may come a time when one partner has expressed an incompatibility in needs and stated that they cannot continue with that incompatibility in place. This incompatibility maybe polyamory as a whole, or a particular person. If the other partner then chooses to change things in order to continue the relationship and become compatible again, they must take responsibility for this decision. They can't then tell the partners that they have to dump that their spouse has "vetoed" the arrangement, they must be honest and say that my spouse could no longer continue our relationship with this thing going on, so I chose to discard that thing in order to continue the relationship with my spouse. That is taking responsibility for your actions