Got my prem recently and been playing lots of it. I've never played the pro, but I can imagine what the game would be like without the upper PF.
In my opinion, you don't *have* to go for the prem/le, but the prem is actually worth the extra $$ (unlike some other titles).
The upper PF is a nice change of pace - it's a bit more floaty than the main PF, and chaining loops is fun and ridiculously fast. Plus that big rubber really gets you wrestling with the machine to keep the ball bouncing on it, which fits the theme well. As others have said, the catapult MB stacks nicely with other features (and it's easier to start than the 3 knights MB).
So you really get more game content out of the prem/le if you see what I mean. Other games - like, say, AIQ - it looks like the prem doesn't give you anything more in terms of gameplay, just some cool ramp/mechs.