1. I would keep it enabled by default, with 100MB max size. AFAIK, every browser has http cache enabled by default,... my firefox defaults to 1GB.
2. If i implemented it it was because it was useful for me. Your mileage may vary.
3. I run Picard often on my existing files, because i edit MB often. I don't buy the out of disk space thing today, sorry. I started to use computers when disk space was a real issue (it was before hard disks on PC), it isn't anymore on any computer Picard run on.