Oh back-up machines are a must! I (and many of my sewing friends have backups for our back-ups) We need machines for home and ones light enough to carry for retreats/classes. We enable each other, can justify almost anything-LOL.
Actually most of my back-ups have come from thrift stores/ garage sales. I only keep them if they are working properly and do most of my own cleaning/lube, minor repairs. And often give one of those machines away when I hear of someone in need or family members. Love, love sewing machines.