You definately do not need a postgrad science degree to sit for the patent bar. I think a lot of people do have them, but not the majority. A lot of people end up getting a technical masters because they have an undergrad that does not qualify them to sit for the patent bar. My law school offers a one year computer science masters for just such people. A BS degree is enough though.
Its true that the patent bar typically does have a very low passing rate - between 17 and 40 percent! However, this last exams numbers were much higher, something like 63 percent.
This high number is very unusual. Apparently they needed to admit a lot of people fast. I would not be suprised if next time the rate drops way back down. Either that or the rate will stay high again and then drop down. I therefore think it would be smart to take the exam as soon as possible, because the high number is bound to come back down.