I think that different people have different ways of implementing the negotiation phase when someone new comes along.
Some couple are fine with anybody, some would like to have a say in things, some others feel the need for a "STOP" button if they feel that things are getting out of control.
I know what you mean, GS - it can smack of control if it is mis-used.
I also think that the need for a VETO tends to moderate as the perceived level of security in the setup increases.
There is one maxim about entering poly that you go at the speed at which everyone is comfortable - this means that everyone needs the ability to say "hold on - stop thing for a while, we need to talk". If this is a good-faith thing, and not some way of manipulating the situation, then to me it seems like it is respectful to allow everyone that ability. Is that a VETO?
This is a debate that has been going on for as long as I have been involved in the poly community with some considering absolute VETO power as evil (and criticising anybody who doesn't think the same), and others who see the need for it in special circumstances.
I know that our need for it has diminished as we have got things more sorted out - now it's more the idea that because we tend more towards the polyfi variety of poly that we want "buy-in" from everyone involved, since they will all be involved with each pother in one way or another. We don't consider that a VETO, though - it's just one of the many things that we have agreed to agree upon.
"Listen, or your tongue will make you deaf." - Native American Proverb