I would search past forums...this was recently discussed in detail. Short, quick answer: No, it is not bad to put one professor's name in SOP, but make sure your research interests are broad enough that you're not limiting yourself to one area (like say "trauma" or "eating disorders," not "trauma in preverbal children" or "eating disorders in males"...it's great to narrow in on your interests, and you will eventually but not at this stage in the game).
This is just in case you are a great candidate and your designated POI has already found several students he/she wants to work with...this way, it will make it easy for the program to accept you if Professor X or Y is available because Professor Z is overbudgeted or too popular with the applicant pool this round.

You can/should detail how this interest came about (for example, "When I played college football, I became curious about eating disorders in males because several of my team-mates struggled with body perception and weight management through binge eating..."), but suppose Professor X worked with anxiety disorders (but not specifically "eating disorders" like the famed Professor Z)...during the interview and in graduate school, you could link your broad ideas with other faculty's ideas, if you were assigned to Professor X or Y's lab, but not your originally designated Professor Z (like supposing you said "Yes, I have this interest in eating disorders, specifically the work of Professor Z, but I recently read how eating disorders can be linked with anxiety, so I'm also interested in Professor X's work in anxiety). CAUTION: Spend time on this and choose your written words carefully, and get feedback from others..if not done well, you could appeared SCATTERED, so do your homework before you write.
This is what "best fit" is all about between programs and applicants. You need to make yourself fit within several professors's interests (to be even more helpful: imagine and find 2 or 3 professors at each school...most dissertation committees have 3 faculty members' whose common interests will link with your future dissertation idea...and don't worry, you don't need to come up with any permanent ideas now..you only need to present yourself as "one who can play (i.e.do research) nicely with others."