I'm interested in learning about ways to combat the keystone effect while capturing landscape/architectural images. Acquiring a tilt-shift lens seems the best solution, but would require me to lug heavy, bulky and expensive gear (such as the Canon TS-E 45mm f/2.8 on a 6D).
In-camera solutions for perspective correction require a shift-able lens or a significant crop of a wide angle shot after the shot. Obviously, cropping will cost image quality, but when the required output is smallish, it might work. Getting the perspective correction perfect in camera is not easy, so you might need to still correct a bit in post-processing.
Software solutions vary in quality, but Pano-stitching software (PTGUI Pro is the most uses dedicated allpication) usually offers superior resampling algorithms compared to e.g. Photoshop. In fact, stitching software offers much more control and accurate projection methods, and it can combine the corrections for keystoning or squaring with corrections for residual lens distortions in one go, thus preserving quality with a single resampling pass instead of two. Stitching software also allows to shoot with longer focal lengths, thus increasing resolution, while making up for the angle of view by stitching multiple tiles. It usually also offers possibilities for removing people/traffic ghosts when multiple/overlapping images are taken of the same area in the scene.
The RRS kit allows to expand the possibilities from single row to multiple row shoots over time. You don't need to invest for the most flexible solution at once, you can do it as the need presents itself (and budget becomes available) over time.