As far as I know, the options for rear suspension adjustment are slotting the stock rear control arm mounting points, and the adjustable brackets (like the ones sold by Datsport). There might be other options out there that I don't know about.
I would say the necessity depends on your plans for the car. How slammed do you want the car? (Seeing as the main reason for needing the adjustments is when lowering the car excessive camber and toe are induced) Do you plan on tracking/racing the car?
I have been driving on my stock rear crossmember with lots of camber and toe for about a year and a half now. It is possible to go without althought tires get eaten up pretty quickly and you don't have the optimal rear alignment. My personal opinion is thar the adjustment IS necessary, but that is because my car sits pretty low and I do plan on tracking and possibly racing my car. I am about to order the Datsport do-it-yourself kit myself, finally!
I'm not sure what your budget is, but if you think you might need it, the do-it-yourself kit with crossmember bushings and control arm bushings can be had for less than $500usd from Datsport. All you would need is a friend with some decent welding abilities.
Hopefully this was helpful.