IMO no one NEEDS a Ca reactor, but If your livestock needs steady high ca and alk, a Ca reactor can be/is invaluable. Without one you will need to do a lot of manual 2 part dosing, and a lot of testing. Dial in a reactor, and all you need to do is occasional testing to make sure things are in check, and occasional adjustments.
Some people are super meticulious and have no issue at all testing and dosing daily. Personally I'm lazy. I'd say normal human lies in between
Here's a quick thread I started not long ago seeking other's opinions re: a reactor on my tank which is all softies with a few clams. I think Greg Hiller's post sums it up very well. http://www.bostonreefers.org/forums/showthread.php?t=55691
I agree with jimmyj7090. It all depends on need. We have a lot of SPS so we run a reactor at about 450. If you have mostly softies and clams, you don't have as big a need.