This is sort of a continuation of Expat's thread about how people can make an ass of themselves by acting like their hidden agenda is stronger than it actually is.
I think one of the things that our dual's do for us is make our own hidden agenda stronger. The strength of the function is totally dependent on their presence and continual support though. While in a way we know that we're only able to be strong in that function due to their input, they generally don't make a point out of it.
This is why illusionary relations are so exciting and perhaps even euphoric in the beginning. They make us feel good about ourselves (assuming the people involved are healthy of course).
I've also noticed that a compliment on the hidden agenda from an identical who I see as being very successful has a somewhat similar effect, though it's not a continual exchange as it is with an illusionary partner.