I think the notion of ideas being worthless is, essentially, an overreaction to the trend of ideas being grossly overvalued at the expense of more practical considerations.

In my mind, this belief is best illustrated in the rapid rise of startups who have an interesting idea, but very little market validation or plans for monetization.

As with most complex topics, the truth is somewhere in the middle. Ideas are not worthless, but they're certainly not worth much. To be valuable (in a business sense) an idea must also be coupled with a strong need or desire derived from knowledge of the market. Add to that a strong plan for execution and you've got something of value.

I'm certainly not suggesting that these elements are always known from the beginning, but for an idea to grow in value, I think these factors need to be taken into consideration.