It depends on how much is costs them to get those 3 tags. In another scenario, they untag a 3 tag guy, and then they have to draft THAT guy in the top 10, and now they've given you an 8th for nothing...
I do agree though that people are not fluid enough with tags
One person's 21st tag might be worth more than another person's 19th tag, and that SHOULD create trading opportunities
It is just a challenging thing to work through in trades though when trying to balance players, picks, and tag values all together. Often times we up doing the sub-optimal thing because all the moving pieces do not click together