I bought the Carmichael Training System (CTS) progressive Power DVD series to do on the trainer in the winters and he uses an indoor 8 minute FTP test (instead of a 20 min test) to establish the different training zones and to measure progress. This works much better for me on the trainer. It is torture to do a 20 min test on the trainer but 8 minutes is pretty doable.
I have easy to access to very quiet country roads but it still is difficult to find a 20 minute outdoor section of roads without interuptions (stop signs, intersections, wind, no downhill sections, etc.). There are definitely advantages to indoor testing.