I'm a designer, so let me flip your question another way:

Is it bad for a designer to use a stock photo if they can't afford a photographer?

Of course not! Will the design look a bit generic? Yes it will. But that's not a big sin in the larger scheme of things if you have a low budget project. Design is about problem solving as much as it is about creativity.

However I will say the following: Just like selecting "off the rack clothing" use care in picking a theme that best matches your project. In fact for an extra $50 you could even get a designer to help you pick something that best matches the project.