Dear Miss Snark,
"Any agent who won't give you client contact info should be avoided at all costs."
"All reputable agents expect to be asked for this information, it's not seen as intrusive or unwelcome."
I have a friend who is happily represented by a reputable old literary agency in the 212. They do not give out clients' names, I recently learned, and they disapprove of prospects' trying to contact authors -- even authors who publish their agent information at their own websites. I don't know how this works for them, but obviously it does.
This is the same mindset as "you're lucky to get us" and "don't tell the other girls what you make for salary". In other words, a little too rarefied for my stiletto heels. Agents are not deities and even the best ones can be total pains in the ass. Better to know going into the deal than discover it when it's really too late.