My Whitemage is currently level 35. And I have Virus cross classed from Arcanist. And from Thaumaturge I have cross classed Thunder and Surecast.
Still have to level up my Arcanist and Thaumaturge (23 and 16 atm) to unlock Swiftcast and one other ability I can use. Probably Eye for an Eye.

Virus might not be the best skill to cross class. But it is useful against enemies who use physical damage. Because it lowers their strength and dexterity when virus is active. So they deal less damage

Thunder is a good DoT to have. I always cast it first on a mob, before I cast Aero. Thunder takes something like 2,5 seconds to go off, while Aero is an instant cast spell.

Surecast is a godsend at times. When enemies hit you. You'll get interrupted easily. When you activate Surecast, your next spell can't be interrupted. Very useful if you need to get that next heal off while being hammered by the enemy. Not that you want that to happen, but occasionally it will. I reckon Swiftcast is even better, making the next cast go off instantly. Very useful once you unlock the trait to make your Raise spell usable while in combat. I believe that's level 28 Instead of having to pray to the Gods that during the 8 seconds it takes to resurrect someone, no one else dies or takes heavy damage. You'll be able to ressurect someone instantly. I really don't like the Thaumaturge class, but I need to get that class up 10 more levels to 26 to get that awesome spell.