Seth Godin did an interesting post (nothing unusual about that) called "the thing about free". It discusses the value of using 'free' in marketing. A summary could be that giving something away for free will win you the attention of your audience briefly, but that we value those things that we pay for, even though the fee may be nominal. This is extremely relevant to the field of online marketing as almost every decent website you see is offering something for free, whether its information on a blog (such as this one), or free newsletters, free evaluations, reports, the list goes on. This is a standard tactic, one which I use myself and encourage clients to utilise.
I've often heard people from various walks of life discuss the value of free things. The conclusion usually ends up placing the same value on that which is not paid for, as advice unasked for. It is about respect, if something costs you nothing, then you most probably won't value it, "easy come, easy go" style.
The difficulty that I find in the current marketplace is that everyone else is giving something away for free, and clients and customers expect it.
This is certainly not some kind of absolute truth, but marketers and sales people always need to take care in their generosity. I like to be generous, and give of myself and my time, but I know that I need to keep in mind that if there isn't some kind of cost to my time, advice or services, then they are probably going to be wasted on the recipient anyway through inaction.