Well, since you're at 30 already, this isn't a question that will be relevant to you much longer (you get Thundercloud at what.. 32?). However, I'll see if I can answer it.
Short Answer: No, it isn't worth using.
Long Answer: Maybe. Pre-Thundercloud Thunder spells have some limited use, but also a significant disadvantage. Unlike Bio or Aero, they aren't instant casts so you can't kite with it really well. However, it does make sense to use Thunder after Blizzard as part of a heal rotation when soloing melee mobs 5-10 levels higher than you (ex. Blizzard>Thunder>Scathe kite>(Thunder again if possible>)Sleep>Heal/MP regen>Repeat). Other than that, the lack of a free double-damage insta-cast proc leaves Thunder spells gimped enough to not generally be worth it, or at best be about equal to straight up casting Blizzard on your regen cycle. Regardless, you should never waste precious MP during your Fire cycle (esp after you get Fire starter), as it will always outshine Thunder regardless of the proc.
Where I see Thunder making the most sense before the insta-proc from Thundercloud becomes a possibility is enmity management. Generally I see low-DPS DOT spells incurring less enmity, though they producing similar damage in the long run (esp. on bosses). This can be a big deal for early THMs in pugs with new players (I see THMs pulling mobs off the tank all the time in the starter dungeons).
So, I guess the bottom line is that pre-Thundercloud Thunder best serves as A) a convenience for kiting, B) an enmity suppressor, and C) a good complement to Sleep when you can manage to squeeze it in.
After getting Thundercloud, however, I find it useful to keep Thunder up at all times. That said, I still never cast it during my Fire cycle unless it procs (free cast), and I just make sure I time it as the last cast before I transpose (or Fire III, I suppose).
Pro-tip: somewhat unrelated to the thunder conversation, casting Scathe immediately before a transpose keeps you from totally wasting a cast. Scathe animation + Transpose animation are roughly equivalent to the GCD, so you get to change your auras up while still throwing out a 140 pot DPS spell. So, it still doubles the cast time of transpose (in a way), but it lets you switch while effectively casting a 93% powered blizzard spell (blizz casts at 150 pot I believe).
Edited, Aug 23rd 2013 7:42pm by GilbertLapine
Edited, Aug 23rd 2013 7:49pm by GilbertLapine