This can be seen as a feature. The amount of topics I'm interested in is much larger than my ability to contribute something even slightly useful.
And dealing with my well intended but often misguided contributions can be a waste of everybody's time.
Anyway, I'm sure that, like with many things, there's no single good way to deal with this. Karma-like could be a way to separate the wheat from the chaff when the community is already built up. When you're starting, when you can deal with "useless" contributions in a economical way, or when you simply cannot afford the bad PR of disgruntled attention seekers, other options have to be considered.