The extra fee applies to everyone, so the advantage would be booking earlier than non-resort guests. I'm not sure what you mean about not selling like hotcakes. I just checked and Rise, FOP and Rat are showing LL not currently available. MMRR is showing a 7:20 pm return time (about the same for FEA).
Genie+ return times for the more popular rides like TT and SDD are already well into the evening hours. When we were there in October, the earliest time we could get for Rise was around 3 pm, and that's the first thing I booked at 7 am.
There are rides that really don't make any sense as ILLs - EE and really any ride at MK. But the most popular rides - the ones people are most likely to pay for - were selling quickly when we were there in October. Maybe it was a different experience when you used the new system.