Welcome to world of warcraft, where people are always delighted to have a mage in their group...
Ok, so you solo and grind. Frost build is good survivability & fun AoE grinding, but I find a fire mage to an exciting way to quest or grind. My mage is level 52 atm, and I've stacked "fiery" gear (+fire dmg). I'm even wearing some greens from the low 30s, but it works REALLY WELL. When I turn on the crits, 2-shotting yellows mobs is not uncommon.... green mobs just don't stand a chance.
** I carry a 2nd set of int/stam gear for when I need to AoE mobs **
Quest rewards always have spirit on them, so they are often just garbage. Those quests are still worth doing, though, for the fabulous amount of XP. I like to pickup green items from the AH. The occasional instance will often yield a sweet blue item that you'll wear for many levels. Save for your mount from level 35-40.