I only use the 7-Day lineups during Spring Training to try and get equal playing time to my guys. For the most part it works well accept that, when entering them, it is really buggy.
I will go through and create my lineups on a sheet of paper and then start entering them into the game. Occasionally when I drop a player on a line, say the 7th spot in the order, it will put him in the 6th spot. Sometimes, it will even pop him up several spots towards the top of the order. I've also had several instances of dropping a player in the Day 3 or 4 spot and it change the lineup from a day or two previous.
Eventually I can get them to stick as I had intended but it is terribly frustrating and adds extra time onto an already time consuming process.
FWIW, I generally go across the lineups entering one position each day. For example, I have a pre-determined order by position (say CF, RF, 1B, DH, C, etc.) I do this bc it makes it easier to spread playing time and to enter. Even though this is non-traditional method of entering the 7-day lineups, I've also recreated this bug just entering players in one lineup at a time.
As a side note, it would make life a lot easier if the AI did a better job of splitting starts on its own in Spring Training. Better yet would be an option to limit plate appearances or total games per player!