You are going to find different opinions on this one, some quite heated and vigorous (you are seeing signs of this already in this thread).
Some object to the terms utterly and refuse to use them, and have issues with anybody using them. some are ok with them being used to describe the relationship configuration, but not prescribe it, and others are quite happy to use it to prescribe what should happen.
Personally, I fall in the middle camp - if it is used to describe the practicalities of a relationship configuration (primaries are people you live with, secondaries are people you don't live with) then it's ok - but as soon as it is used as "but you're my secondary, therefore you can't move in with us" then I have a major problem. I also don't believe in using it to come up with some "hierarchy of affection", either - that's horrible, in my opinion. Franlin wrote an article on this somewhere but I can't seem to find it.
I tend to avoid the terms completely, only using it when it is necessary to get very specific about describing a current relationship configuration. I won't refer to any of my partners as "my primary" or "my secondary" - just don't like the terms at all in that context.
"Listen, or your tongue will make you deaf." - Native American Proverb