Steam Machines are marketed to Mr and Mrs A.V. Ridge and their 1.8 children. They are going to expect the same basic functions and flexibility as all other consumer electronics. Currently Steam falls very short on a basic expectation all consumers will have.
If they already have 1 steam machine and 50 games, and purchase a second machine they will believe that when the first machine is in use the 49 other games will be accessable on the second machine.
Steam does not offer this functionality, and without it VALVe will fail in the consumer electronics market. VALVe must sort out the Steam Family Sharing before the release of the Steam Machine.