An interesting thread, raising some points that I'd never considered before - like how the covers were all pre-1962 songs which they played in Liverpool/Hamburg, so the standard of the covers stayed at one level while the Beatles' own songwriting and recording improved dramatically from album to album.
I personally don't think BFS is any worse than PPM or WTB, though I admit I'd probably put those three behind all other albums apart from YS. I know Mr Moonlight gets slated but it has never bothered me - certainly not one of my favourite Beatle tracks but I can think of quite a few I like less (Honey Don't being one of them!). I think Kansas City and Rock n Roll Music are great rocking songs and I've always liked Words of Love (probably because I like Buddy Holly). Everybody's Trying to be my Baby is nothing special but it's not terrible either (and they needed a George vocal on the album to please the George fans). Leave My Kitten Alone is one of my favourite Beatles covers and I think it would have gone well on BFS but they already had a John rocker with RARM.
I sometimes wonder if the reason that BFS is never higher up peoples' lists of favourite albums (including my own) is not because of the covers but because the original songs are predominantly "down" songs - I'm a Loser, I don't want to spoil the party so I'll go, Baby's in black and I'm feeling blue, I nearly died, One day you'll find that I have gone, I'm feeling blue and lonely. The lyrics to Every Little Thing are positive, but the music does have a downbeat feel. Only Eight Days a Week out of the originals could truly be said to be an upbeat song. I think the covers (most of them) at least inject some fun into the proceedings, even if they are beginning to sound outdated when compared to the originals.
I do think that BFS is something of a step backwards for The Beatles after AHDN - and the covers contribute to this - but given the year that they'd had it's not a surprise. They must have been struggling for original material and wanted to get an album out in time for Christmas so had to pick a few of their favourite oldies to make up the numbers. Plus it's an unfair comparison anyway because AHDN contains two A-sides and 2 great B-sides. If they'd dropped Honey Don't and Mr Moonlight from BFS and added the single tracks I Feel Fine and She's a Woman it would have made it a much stronger album (and more upbeat), yet they were trying to give value for money to fans by not putting their singles onto LPs.