On paper, this "rule" can be good, but in practice, its only good for people who are abusing it above limits.
1. You never know if people call their MS the different one that they use; if they really use it, play it and have solid gear.
for example,
I can play Holy Pala with 5.5k gs; call my MS as a tank with 0 items ready to play it. Simply because i have no items to drop from said raid to my holy spec. Is it fair to tanks that really need those items? Not rly
2. Like in the post above, someone can be fully geared and still roll for a friend by changing his MS.
3. If someone has for example, a healing spec and there is a space for healer, but he plays dps spec just to play something easier with low effort, thats stupid.
4. Nobody is tracking that perfectly, i had balance druid who called his MS feral, but still rolling balance items(caster tinkets and so on) like its his first spec, BECAUSE hes balance right? WTF
EVERY SYSTEM BASED ON HUMAN TRUST AND BRAIN IS NOT GOING TO WORK GOOD; you can't disagree with that, people will abuse everything. I am 100% all the people above abused or abusing it to this day, that's why they are defending it.