It's all about total volume. I train every muscle every 48 hours for 2 months and then take 2 weeks off to fully recover. I start off light with 15 reps at the beginning of the 2 months and by the end I'm lifting so heavy I can only hit 3 reps.
Though i do full body workouts every other day, the total volume is roughly the same as someone who hits each muscle once a week.
Example:
Every 48 hour routine Quads:
Monday - Squats (3 sets of 10 reps)
Wednesday - Squats (3 sets of 10 reps)
Friday - Squats (3 sets of 10 reps)
Total weekly for Quads = 9 sets (90 reps)
Once a week:
Monday - Squats (3 sets 10 reps) Leg Press (3 sets 10 reps) Leg Ext (3 sets 10 reps)
Total weekly for Quads = 9 sets (90 reps)
There's no way you could do the 'once a week' workout & volume every 48 hours for every muscle group, that would rapidly lead to over-training. Break the total volume a week rule and you will become over-trained unless you eat like a horse and have the recovery ability of a bodybuilding god. (or you aren't lifting heavy enough)
The idea behind hitting them every other day is to have the muscles constantly repairing and growing
Hitting them once a week causes a maximum of 72 hours worth of damage to repair. That leaves 92 hours a week when a muslce isn't growing!
Though hitting them with just 3 sets every other day doesn't cause the same damage or stimulus to grow as a high volume once a week workout, so it's down to individual's to decide which works best for them. A big bite or little nibbles.
Though believe me you can have pretty big nibbles when squatting every session
9 sets of squats will release more growth hormones in a week than just 3 sets
(there's more to it than just volume, there's intensity aswell but I'm off to bed...)