It's true, you can use a jack between the frame and brake rotor, and you'll accomplish the same result. But if you detach the sway bar - 2 bolts on either side, which are easily accessible - you can install one side just be pushing down on the axle. The other side you have someone stand on the brake rotor while you install the spring, or you could use a jack like tommudd. If I were to do the rear again, I'd do it the same way. It's a matter of 'mechanic preference.'
I used the compressors successfully for the front, but ran into two problems. First was the issue of where and in what direction to use the compressors on the spring. I had the top plate interfere with accessing the compressor hex head. I already had the spring compressed, and the top plate on and tightened before realizing I was going to have a problem getting the compressors off. I loosened them as much as possible with a GearWrench, then knocked them over to clear the top plate and used the socket/impact to finish. I got lucky the first time, but ran into the problem on the other side.
Second was the issue of the top dust cover for the strut - I left mine out. Not for any reason other than the spring compressors left no room for the covers to be installed. So, I have front struts without protective covers. I feel the covers are important enough that I will have the front wheel bearings serviced at the dealership, and have then also remove the struts and install the covers. I know it was stupid to leave the covers off, but I was going to Death Valley Friday, dammit - and I already had everything apart and the rear lift completed!
I was against the clock for my Death Valley trip, so I used the compressors to install the struts. However, I recommend having a shop do it. I'm usually a pretty safety-conscience mechanic, but using the compressors just felt too unsafe for me. I did it because I wanted it done NOW - but I will be very reluctant to use spring compressors for this again.