I recently encountered the opinion that we should be willing to meet with anyone who asks for a meeting, because we never know what kind of opportunities will open up.
At first, I thought "maybe". I could see where you could want to be open to anyone who approaches you. As an avid networker, I really shouldn't turn down any opportunities.
Then I came to my senses.
We all have a lot of demands on our time. While we can be open to serendipitous meetings, we can't allow others to dictate how we choose to spend it. If I met with everyone who wanted to meet with me to tell me about their product or business opportunity, I would have a full time job listening to them. Some day I'll tell you about Jay Petersen's diabolical plan to address those who want to pitch him.
There are obligations we must meet, including our employers' expectations, the needs of our families, and a little bit of sleep. We've got to have priorities that also include those people we have already established relationships with and those who are our targeted markets.
Can you expect to demand someone's time and be disappointed or upset when they don't oblige you? Good networking skills require us to make an investment in the other person first, before we can ask for something from them. Thinking that our need for their time indebts them to us is akin to taking a negative response to a cold-call personally.
If you want someone's time, you've got to make sure there is value in it for them, not just for yourself. That is a fundamental rule of networking. Give first before you ask for something as precious, fleeting, and irreplaceable as their time.