The problem I have with leaving stuff in place when you do the 303 is that in some cases you can only really reach maybe 80-90% of the pleather surface - for example parts of a belt or jacket that are folded under, the inner lip of boots etc - so those areas that don't receive direct application of the 303 so can still potentially fail.
I did several Indy jackets recently and there were a lot of areas on the jackets that were folded in on itself - underside of collars and tabs, deep jacket pleats, seams folded inward (ie where zippers go) - and you really would have missed a good 15% of the pleather surface if you didn't take it off the fig, and crumbling in these areas is probably as fatal to the jacket as a visible surface crack.
It's a staggeringly frustrating thing that these $250+ items have parts that in some cases only last 5-10 years, and if you're talking a character that has a lot of leather on their outfit, you're in trouble.
And I guess the 303 question (I've done some, but I have something like 150 figs all up and maybe 1/2 have at least some pleather parts) is still an open one - just not sure that anyone has used it long enough for us to really understand if side-by-side a person who used it kept the pleather intact and the person who didn't had it fail.
Bottom line for me is that I just don't think its possible in the real world for me to coat probably upwards of 300 individual pleather items of varying sizes, many partly hidden under coats and other stuff every 6 months or so, which is what's necessary. It's just countless hours of stressful, joyless work that has to be done over and over (303 would be awesome if it was a "one and done" process.)