I had much the same thoughts for my second shammy to support the lead melee shammy in my two box setup. I wanted her to do a little damage to the target, but to throw an occassional heal spell on the melee/tank.

Now the above mentioned "modifier" solution might work, but you still have to hit that modifier. The other suggestion to use [harm] and [help] modifiers means that you have to change your target constantly. I also found in testing that putting both these in one statement ended up with me casting the harm spell until the enemy died, and then casting the heal spell...

I was really looking for a spam solution. What I came up with was to use two /castrandom spells: 1) attack and 2)heal_tank. By putting multiple commas(,,,) at the end of the /castrandom statement, you are basically telling it to have a couple options of "doing nothing", which lets the first /castrandom attack statement do nothing, and allow the second /castrandom heal_tank statement execute because the first one didn't reset the global cooldown giving you a "spell not ready". Keep in mind that you are using the RNG, so it might actually have instances of "doing nothing" for both statements, but I just look at that as a mana saver! LOL

Trying to do a /castrandom [harm] spell1, spell2, spell3, [help] lesser healing wave ???