I'm currently developing an interactive Flash presentation to run on pharmacies touch screen monitors. You've seen these before...
Now, I'm not a designer. Not very good at it either. I'm "just" a programmer. But was given a certain creative freedom for this project as long as I use the materials the pharmaceutical gave me.
When a designer tells me my creation sucks, I take it as gospel. If he says so, it's because it sucks. I go back and change it. I'm not trying to defend my creative process or my professional honor; As I said I'm not a designer. There's nothing to defend. What I may do sometimes is express my opinion: If my aesthetic standards tell me their alternative solution sucks, between both sucking options, I may think mine sucks less. But that's it. The designer has the last word. He just learned my personal opinion.
So when I tell a designer that what they want to do is technically challenging or simply not possible (within the current development time window, or budget), I expect them to understand that I'm now talking on my grounds. That's my domain. He should take it as gospel and either increase the budget or give me more time (depending on which). Concerning programming, what I say goes. There's no debate. I may be willing to hear his opinion though. But, as before, that's it.
Because I'm stupid, I tend to respect other people's space and the knowledge they acquired during their careers. Unfortunately I'm not that lucky finding others who do.