I must admit that while I could see the merit in them, even when I was younger and had more time on my hands I didn't really bother with Platinum/HGSS's frontier much. It was always far too intimidating and felt like a level I could never reach and furthermore even in the Battle Tower I often lost for luck reasons outside of my control; and I can only imagine this is a big part of what contributed to GF realising not a lot of people played these. They're very interesting in concept, but in execution they need a bit more of a draw and something to ease people in. Maybe there are lower versions with less penalties, but then more 'competitive' levels the further in you go that much more resemble the Frontiers we're used to. A little bit like Battle Tree's current 'singles' and 'super singles', but a little bit more thought out.I frankly didn't like the Battle Frontier much either. Outside of the Battle Tower and Battle Factory (which I both really like; it's a shame the latter was also cut outside of USUM), the other facilities are too gimmicky to be enjoyable.
(Not to mention the Battle Factory was also a nice way to get BP when you didn't have a competitive team yet)