so I can only use runes of my Primary profession?

I'm a Warrior/Mesmer (don't see many of these, I think) - and I bought a mesmer rune of minor domination, only to find I couldn't apply it to anything. So I guess only runes from my primary profession work? Seems like it might lead to more diverse characters if you could use runes from both professions...

Yeah I think it really would but it doesn't work that way...

Yes you can only use runes from your primary proffession. This makes you think a little harder about which class you will play primary. In reality, all it does is make it so your secondary proffessions cannot go beyond 12 in an attribute, not a huge deal.