Yes, seasonal trading can be a very valuable tool.
To my thinking, it is best used in conjunction with fundamanetal/technical analysis, not as a standalone trading tool.
Many commodities do exhibit strong seasonal tendencies. Seasonal charts are great visual tools for identifying seasonal tendencies during periods of the year, but it is best if you can also get the "numbers behind the charts" - that gives you the probabilities, max risk, avg drawdown, profit targets, etc.
Putting all of the above together can certainly help to improve your results and hopefully profitabilty.
As for the request for recommendations for services, I would not consider these recommendations, but here are 3 services I'm aware of. I would compare to see what suits you best.
Moore Research:
www.mrci.com - charts and some analysis. they provide monthly strategies as well
Seasonex:
www.seasonex.com - lots of numbers and otpimization, but no charts
Futures Research Corp -
www.futuresresearchcorp.com - charts and numbers as well as 60 days of strategies