Checkpoints would be good for practicing certain phases, but that's it. Practically they wouldn't work for reasons mentioned above, and if they were to add checkpoints now, it would just invalidate the effort of those who cleared the fights at their hardest even more than they already have.

Most of the coils can be cleared without a static/putting your life and limb on it. We have several people with heavy work/children/family in the FC who still managed to get a clear on all the coils. If you're not willing to put enougj effort into beating the fights, maybe they're just not for you? You can just forget them and continue plating happily, knowing that those fights are perfectly optional.

Practice mode would be good though.