I always ask for a un/pw to see the members' area. The reason I ask is because I like to see if the site is worth heavy promotion or moderate depending on the consistency of updates. I like to see if they have a full photo, video and webcam archive for the members.
The only three reasons I think I don't get access to review the members' area...
1.) They have a shitty members' area.
2.) They don't trust me.
3.) They think they are the only affiliate program on the Internet and don't meet their conditions.
a. You have only been an affiliate for a week.
b. You haven't sent more than 10 sales to our program.
Affiliate programs that gave me access have had great members' area and consistently get my traffic.
Affiliate that didn't give me access or just ignored me got no traffic.
The best example of affiliate support with automatic members' access to all their sites is
HUNDIES.