Which bushings? Maybe in the spiders has no affect on anything else, however in most cases replacing anything in the diff's affect either the clearance in the gears or a bearing pre-load?Replacing the bushings isn't much different than knocking out other bushings or removing ball joints. I'd guess the labor involved is greater for replacing the rear subframe than replacing the bushings alone. In both situations, you still have to pull the differential out to get 2 (or 4??) more bushings but the aftermarket option leaves the subframe in place.
On the ball joints, the two Nissan's in my sig you can only buy the OEM lower control arm assembly. OEM Nissan MSRP is $142 each Do you really want a tech at $200 an hour driving out ball joints for $142? Also, if your ball joint is shot odds are good your control arm bushings are as well.
Even if you disagree with me, its far from from a Nissan only problem.