If you think about marketing: we are all basically providing free marketing for Arduino. Arduino is a platform product that greatly benefits from our discussions.
Everyime somebody shows off with some new Arduino application it is likely to get featured by Hack a Day or other blogs. The cummulative effect is that Arduino gets more attention.
Arduino even benefits from discussions about better alternatives. Because in the end absolute beginners will always default to the "real" thing because the price differences are not that big.
And Arduino itself is not the lone inventor of this great AVR platform. It started with Atmel providing really low cost development tools, people writing the AVR-Libc and others providing gcc, avrdude and so on. So Arduino just created some nice package that lowered the entry hurdle (both price and diificulty levels) even further. However one can come to the conclusion that they also benefit from developments of others.
So I would expect that it is absolutely OK for members of the community to market their own stuff in order to benefit a little bit as well. Even more so for active members.
Udo