I'd say keep it up to date if it's going to be kept and ditch the system if it's going to be ditched.
There are some Future Interstate routes that are not yet in active systems, such as FI-905 in California, a few FI-49 segments in Arkansas, and I think one in Tennessee that includes a state route signed only as an FI and so isn't in the HB as an active state route. The system is worthwhile for those cases, at least.
As for the ones that are concurrent with routes in active systems (usually US routes), no biggie if one isn't added to the FI system, and no biggie if one isn't removed from the system. Once we have active state route systems for all 50 states, we can figure out at that point whether the FI system is worth keeping around.