Focus is the key to entrepreneurial success. You can't be all things to all people. You have to develop your core competencies and competitive advantage. I've heard these things. You probably have too. If you've ever started a business though, you've probably ignored them all.
In the early days, there is always a tendency to take on customers, regardless of the type of work you are asked to perform. Why? Because you need the money to keep your doors open. Such customers can be salvation to a startup, because they may introduce you to markets you didn't consider. Or they may cause you to lose focus and stop developing your core products, so that you end up playing in markets you didn't want to be in. This, in essence, is the bootstrapper's dilemma. How much do you stick to your original plan, and how much do you break the plan to get customers and cash?
Zen Entrepreneur explains it in depth on his blog.
This is an interesting question because you have two conflicting demands that are coming at you week after week, month after month:
1) you need enough cash and profits to keep the company going, and
2) you have very limited resources so it's important not to spread yourself too thin.
In fact, these two demands are at the core of what I call the Bootstrapper's Dilemma: If you have limited cash and need to keep the company going, you are likely to take any sales/revenue you can get, even if it's not in the area of your focus. Furthermore, you may discover that the real opportunity is in a market/area that is adjacent to what your first guess was. But if you don't focus, you are not likely to make much headway in any of the areas that you are attacking.
Some people want to gamble big. They go for focus and either win their market or go out of business. Others will do whatever it takes to keep the doors open and live to fight another day. The path you choose depends on your particular goals and your unique situation. If you are struggling with this type of decision in your own business, go read the whole post.