Assorted comments:
Good compensating doubles DO have good F-sides, especially with an ML bell throat, e.g. Kuehn, Kruspe, Alex, Yamaha - probably 80-90 %, of the playing quality you would expect from the F-side on a good full double. There are advantages to lighter horns (see below), however good full doubles obviously do have better F sides still.
I tried a couple of the Cornford double-compensated triples several years ago when the design was new. Unfortunately they both had a noticeable resistance increase on the low F-sides, especially when using additional valves.
F-extensions on Bb singles and descants make a lot of sense. You can also use combination fingerings to get E, Eb D and Db crooks (with a little right hand correction) and these can be used for both their tone colour and hand stopping (as well as the obvious pedal notes). Since I added an F extension to my Bb single, I never took it off again!
I used to play on a full triple. Compared to a normal set-up, the extra weight makes it easy to produce a powerful sound, but the corollary to this is you have to work harder to produce a lighter sound for chamber music etc. Playing a heavier instrument also tires the embouchure more quickly, so if you have a full triple you will feel the need to use an f-alto more often due to the mass of the instrument!
Since getting a lighter weight full double (Kuehn) 6 years ago, I haven't ever felt the need for an f-alto, even for extreme high register gigs. A lighter horn can still produce a powerful sound when required, but gives increased flexibility and more tone colours for chamber music settings.