The way I see it, the more frequently you can hit a muscle without overtraining, the better you're going to grow. However, goals experience, exercise selection, and overall volume and intensity all play a factor. As brikis said, a beginner can get by (and often make better progress) with training 3x a week. As they get stronger, intensity and/or volume needs to be adjusted. This is why Bill Starrs 5x5 and variations of it either have you do light squats or front squats one day out of the week. Or look at HST where volume is very low on any individual day, but the body is still hit 3x a week. I feel there are drawbacks though which is why I don't use it in my own training - as you start to get pretty strong the first movement will always effect exercises that follow. Especially when it comes to heavy squatting and deadlifting. Also, there's actually been
a lot of research on optimal volume per muscle group. Their conclusions appear to show anywhere from 30-60 reps, with diminished returns as volume got higher. Trying to get 30-60 reps per muscle group in a full body workout may end up being a long workout.
I don't think training a muscle group once a week is optimal either. It definitely works for some, but people have to realize just because it works for some doesn't mean it works for the majority. You'll always find some method that will work on somebody. This method of training is popular among pro bodybuilders (read: drug users) and people have to realize drugs and genetics do change things. Many people on drugs and/or have great genetics often grow in spite of their training, not because of it. You also can't look at how top level competitors train now, often what they do now isn't the same they did to actually get there.
When people end up training a muscle group once a week they also tend to do way too much volume, which is no wonder they need so long to recover. The thing is, as research has shown, too much volume just hampers recovery while not helping growth anymore. You need enough volume to stimulate growth, but not more. Remember the saying... stimulate, don't annihilate. Most people don't do this. There chest day usually ends up being something like Bench, Incline Bench, decline, flyes, cable crossover, etc. If people ended up dropping it down to about 2 movements and concentrated on getting those stronger and get volume under control, they tend to grow better. The one thing this type of training is good at is making you sore, but this should not be the goal of your training.
Also many of the responses to to training such as an increase in protein synthesis, IGF-1 levels, mRNA levels, etc. all return back to normal in about 36 hours and recovery can still take place even if the muscle group is loaded again. So the idea again should be to hit the muscle as frequently as possible while getting enough total volume, applying progressive overload, and using an appropriate intensity to allow enough of a tension and fatigue stimulus while still allowing plenty of recovery time to actually recover.
For most people, I feel optimal frequency lies in the middle, which would be something like every 5th day or 2x a week. Usually this means some sort of upper/lower split, but there are other ways to acheive this frequency as well. Just look at Layne Nortons Power/Hypertrophy split. Volume is quite high and I wouldn't suggest other people try it right off the bat, but the frequency is right in line with what I suggest. Look at Westside templates which are pretty much an upper/lower split, or DC training which you end up hitting everything every 5th day.
So... that's a long way of saying I think what the OP is doing is optimal and as long as his volume isn't excessive (or not enough) and he's getting stronger, don't mess with it.
EDIT - I know this is already too long, but another thing to keep in mind is deload periods. Whether you do it every 4th week, or every 6-8 or if you just drop volume or intensity is up to you, but some sort of deload should be a normal part of your routine. I feel this is one of the most important factors most people don't take into consideration and it'll allow you to drive intensity/frequency/volume up for a bit while not running yourself in the ground because you're doing it for months on end.