Even judging it on armor type is flawed. As a healadin, I have the luxury to be able to use all four armor types. However, it's not always the case that plate is best in slot - for example, the BIS chest in T7 content is a mail one from Maly-25. Personally, I do pass for other people who can use it first - but I would be annoyed if I was *forced* to pass. I pass because I understand that it helps guild progression (I can more easily get a high quality substitute to fill in until I can get my BIS piece), not because Blizzard forces me to.

In Ulduar, I may be forced to *not* pass because of Blizzard seemingly giving every easy-to-get (ie. non-hard mode) healadin piece of plate mp5 - a stat which is mostly unwanted by us. As such, I may be forced to go after some elemental shaman gear instead (that has the ideal crit/haste combo) purely because of a lack of plate options. We'll have to wait and see, but it's a real possibility.

Basically, you should never rule someone out because of armor proficiency (by default; within guilds or raid groups you're free to setup whatever rules you want to enhance progression).