This is for big plants... for people who need to push the boundaries of what is possible to get more power. Crazy people, if you will. And because it's possible. For science.
You say that 300 million EU is a totally ridiculous amount? I'd like to tell you it isn't. It's a totally reasonable amount. Some SMP servers have such enormous solar arrays that people are pulling in over 4096 EU/t in a completely harmless manner. That generates some serious, serious power (and lag). If you are going to have something that generates power in a dangerous manner, it better provide benefits that are higher than what the safe options have. That is why people use nuclear reactors, because although they can be dangerous they can also generate a huge amount of power.
To put it in perspective, 10 million EU will buy you 10 UUM. That is about 1 diamond. 150 million EU (probably around the cost to run the reactor) would have bought you 150 UUM (I doubt you'd have the scrap to make this more efficient), about 16 diamonds worth. So the Rift generator provides you with about 16 diamonds worth of power over the course of one VERY hectic day. Not unreasonable.
Also, hopefully you put this thing away from other peoples' houses.