I gave both you and Crone5 a thumbs up. I actually do that when there are well thought out posts that I mightn't agree with. It is the exchange of ideas that I appreciate.
In my view there are 2 schools of thought. Diversification is a good thing and should be encouraged. The second which has done the rounds over the last few years is that 90% of your revenue comes from your top clients. You waste resources and time servicing the over 90% for little marginal gain. You are better to discard that 90%, service the over 10% and look to increase your service offering and cement the relationship into a dependent one.
To be honest both are equally as valid, as long as the execution is correct. Get it wrong in scenario 2 and you are buggered. In scenario 1, spend to much time on under performing clients or over servicing them and you miss growth opportunities. You may also lose top clients, as your resources are fully allocated when they want more from you.