I haven't tried Nidhogg yet so no idea about that fight, but based on my previous experience, if you reach a phase without failing before that, what is wrong with wanting to train from there?
I don't need training in phase 1-4 if I make it to 5 without trouble, even if I did not wipe 500 times in each previous phase to "get used to it" or whatever. Some people learn faster than others, so what's wrong with it?
It takes me one, at most two wipes to get the mechanics of a fight down, up to 4 to perfect my timing on selfbuffs and the like (again, based on previous fights, no idea how Nidhogg will be for me). I don't see any reason to keep wiping in phases I can already do without fail...

What you are actually complaining about is people joining who don't know what to do in previous phases. This has nothing to do with the amount of training they got though, simply with them wanting to get pulled through to get a win without putting in the actual work. I joined a Sephi EX clear party a while ago, and when I asked 5!! of 8 people admitted that they were entering the fight for the first time. Their expectation being that 7 others would be good enough to clear so them failing wouldn't matter, right?