Unless you plan to thief *a lot*, I'd not bother with therapy series.
I have it maxed atm on my char and I use it soley for thieving. If I need to go afk, I'll just warp to town (+, it doesn't stop giants and champs, so can't be afk too long). For trading, it's good to have if you are doing a big run and everyone needs to stop and take a break, but that isn't often.
There are 2 main uses for therapy for thieving:
1. Popping
2. Lazy Scrolling
For popping, you just sit somewhere and let your NPCs rampage everyone near. Of course you are a sitting duck for that 5 mins its cast, but you can use that as a "base" and return there after you pop other traders and get more loot.
For "lazy scrolling", let's assume you have a good 4-5* load. You will be spawning some really though NPCs, reds and oranges. Any build you will have trouble with. I'm str and I have to be really careful b/c JM likes making me spawn 3+ cold bladers. The idea is though, find somewhere safe, cast therapy, and wait for your scroll:) If you cast scroll first and then therapy b4 that next spawn, you will be safe the entire time, scroll is about 5 minutes too.
The only thing you have to worry about are the champs that you might spawn. Those are the only ones though, don't run out to fight it! Just let the others disappear, then take it on if it's a archer outside of the circle. If it's in the circle, make sure you don't do a AOE attack that will aggro the others near.
The best payoff is of course when you do double spawn with that 4-5* load. You'll just be sitting there, the area around you is a minefield
Aside from that, there's no other big use of therapy that makes it worth having. Res is good in that series, as well as the heal and the status cure, but not used much.
I have decided to use item mall to take away all my force, I'm thinking about making a blader rather than a glaiver, so I need all the SP for cold as well. Cold isn't going to be my main imbue, but I want to be a real tank

For my level, I'm in the 45-50 range to give you an idea of the experiences I've had.