obviously the more you train the better you get, but I think it depends very much on your personal skills and aptitude what you will get out of the training. In my dojo, we have four classes per week, and most aikidoka come only twice. Myself and two or three others come four times, and I don't see really a difference in our advancement; apparently we just need to put in more effort to achieve the same.
This said - it does not bother me very much, because I think if I train the double time, I also have double fun and more lessons to look forward to. If I also calculate karate lessons and aikido seminars, I'm doing martial arts between 6 and 10 hours per week, thus corresponding to a full working day, but that does not make me a master very quickly. I'm also an engineer, and if I work one day per week for 20 years, I'd certainly not become one of the best ones in my area of expertise
Paraphrasing Marx, everyone trains according to his needs, and everyone advances according to his capacities...