As it happens I have a bike which has a disc brake up front (avid BB7) and a cantilever brake at the rear (Avid Shorty Ultimate with Kool Stop dual compounds). Housings are compressionless and the cantilever is setup for maximum braking power. I prefer to use the disc up front because it just performs better. The cantilever at the back is somehow 'laggy' and it takes a fair amount of lever pressure to get proper results. The mechanical disc brake achieves more with less effort. It's not even a front brake rear brake difference kind of thing because I've had full rim brake bikes with cantilevers/others and they performed just as poorly up front. The end result is that turning left sucks because I have to signal with my left hand and brake with my right, which controls the rim brake. Yes, I could go back to full cantilever so I'd have the same amount of brakes at both hands but then turning both ways would suck. It's surprisingly unstable to ride one handed when you have to really squeeze the lever with effort.
Then I have another bike which has hydraulic discs front and back.
The difference of hydraulic disc brakes when comparing to rim brakes is like comparing the brakes of an 80's Camaro and a modern Volvo. Both get the job done (kinda) but with one getting the ABS to rattle is achieved accurately and effortlessly with one toe while with the other you need to merge with the seat to lock up one wheel on wet tarmac.
First of all the hydros are more powerful by far as in stopping with them is easier. I know the physics and once your rear wheel is off the ground that's it. But what I mean is that braking is easier in that it takes less effort to achieve full braking power but the brake is still more accurate than other brakes of the same braking power (full length vee brakes get fairly close). So to get my rear wheel to rise I don't have to have visible abs on my palm. The end result is that I can feather at the limit of braking more easily than I can with any rim brake or with mechanical disc brakes. It's weird that the feel and security I get from the hydro's with 160mm rotors is better than I got from a mechanical disc brake with a 203mm rotor.
With the hydro bike I don't have to consider which way I signal because I get the same performance from both brakes. Safer, easier and braking doesn't make me as unstable as braking with a cantilever will.
Now I'm not sure if modulation is the correct term but it might be. Hydraulic disc brakes have plenty of it. Cantilevers or other rim brakes not so much in my experience. The more strength one has to use the more one loses accurate motor control. That in turn makes braking less accurate. Every rim brake I've ever used has required more effort to achieve the same braking power than disc brakes.
Modulation is probably also a bit misunderstood as a term. But in the end it really comes down to how accurately one can control one's brakes. Lever travel has nothing to do with it. One could have zero lever travel and have great modulation whereas one could also have a full range of lever travel and and have poor modulation. As an example, highest end racing cars which do not use brake assists (Formula one) have almost immovable brake pedals, yet the brakes are accurate enough for pretty high lever racing.
For me rim brakes are an inconvenience I'd rather not deal with anymore. And I do feel safer with disc brakes.
Whether one should adhere to the N+1 rule? Yes. Yes one should.