If a pupil has a finger 4 that 'collapses' they will find it hard to control the 5421 fingering. It's important that the fingers are evenly placed over the keys to enable a free wrist movement. My rationale is to teach a fingering that will serve a pupil well when they get to big, difficult chords later on. For example, the transition from a D minor arpeggio struck all in one go, to a D major shape using 5321 is a lot easier and safer going from 5421.
It's easier to remember fingering groups as well. 5 4 for the white minor 3rds, 5 3 for the white to black key major 3rds etc.
I recall in an old Trinity book that they absolutely insisted on the standard fingering printed in the exam book. The most recent Grade scale books the AB published in the 90s show an alternative 3/4. I don't agree with this but having said that I once had an adult pupil who absolutely refused to use his finger 4 - so not a lot I could do about that - it can be uncomfortable for some pupils - for those with very large hands or who haven't got much stretch between 5 and 4 - like my adult!
Karen - it depends a lot on pupil hand shape - but as a general rule if the LH finger 3 is forced too far to the left using the 5 3 2 1, restricting free movement, then try the 5 4 which I believe is correct - if the pupil can manage it
Perhaps the AB are being too lenient!
Ow - my wrist aches now trying all this out on the table in front of me
P.S Chocolatedog - I think a lot of adult tutors go too fast. I've got the Hall, the Alfred and now the Pianoworks all on my shelf and I am hoping to use them all for new adults in the future and see which works best. Running favourite at the moment is the Alfred and Denis Agay 'First Year Piano'.