well mr day you could tell my story. honestly, i think people can do more than they think they can. i mean, i am no elite rider or anything, just a 42 y/o enthusiast from wisconsin who got them, put them on, and kept up mileage for an IM 10 weeks from the event with really no problem. the 100 mi day 3 ride is well known - but i also did 2, and 3 hour rides, and two a day 3 hour rides, 2-3 hour hill repaet days, and whatever in the first weeks i had them. did i say that i believe very strongly that people can do more than they think they can?? don't take it from me, ask anybody who just got out of boot camp in the marines. we hold more untapped potential in ourselves than most people know. the biggest caveat is they need to forget the speedometer, and instead look at their HRM while adapting. as you say the speed will suffer while they are building the muscles to PC, but fittness will not.
as for the issue of exclusive use i would say the same. i would make an anology with weight training. that is, if you are starting weight training you ideally lift with proper form. most times using proper form means you will, at first, be lifting less weight for less reps than if you "cheat" the form. PC's are proper form ! if you weight train correctly 2 times, and then incorrectly one time just so you can say you lifted xxx weight xxx times who have you cheated? it takes some degree of understanding, patience, and ....i don't know...self confidence(?) to let yourself be worse so you can get better - not everybody wants to do this.
finally, on the topic of switching to regular cranks to "work" the quads. i know some coaches are big on this. i believe it is simply a standard by-product of the technique of easing up the quad's drive so the HF's can physically keep up. this will ease in several short weeks and one can resume all the hammering they want. OR if they do not want to wait they can go do seated hill repeats at a low cadence, or simply do it lemond style and select a big a$$ gear and grind along the flats. the idea that the quads suffer during PC adaptation is simply not true, if you use your head, IMHO.
for me, having now adapted to the PC's well enough, i can't see W-H-Y one would not want to use them on a ride, provided one has them. contrary to the oft alluded point that one would be better "just working it the old fashioned way" or whatever - PC's are WAY more work than regular cranks. they are the antithesis of a "magic bullet" or "training gizmo". they make you work like a mutha every single pedal stroke. since coming to tri i have learned to appreciate the need to get the most out of every workout, and PC's help you do that quite handily. to me the question is, if you have them, what benefit is there in NOT using them? none that i can see, apart from the occasional ride where your will be at your limit to hang onto a wicked fast group or something and you can't afford to make one pedal stroke in err - or carry one additional pound.......a virtual race , as it were.
the other point, not so related, that i see is the cost issue - they are costly, yes. but. so are p2k's, and 404's, and TI workshops, and whathave you. i am of the opinion that a $1400 tequilo will get you over the finish line as fast as a $2100 whatever. a $1200 set of wheels might gain you 8 min in IM bike split, but in 10 weeks PC's gave me almost 4 times that in the run alone by my claculations. i am a 90 min IM swimmer, and i am pretty sure i could spend 800 bucks on coaching and get NOPLACE near that kind (26 min) of improvement ever, let alone 10 weeks. well and good - we can all spend on whatever think will be the most fun. the thing is, seldom do you see people come in and bag on TI workshops, or 404's, or p2k's like they do PC's. i myself used to do just that, and for the life of me i cannot remember why. tim