The only problem I see is with your career goals. I don’t know of any psychologists who JUST do research. I don’t know of any psychological research which takes place outside of a university setting, and most universities don’t hire people just to be researchers. They are either faculty, which means that they teach, or they are clinical, which means that they do counseling. You may know of jobs that I don’t, but if I were you I would spend a lot of time making sure the type of job you want actually is common before putting a lot of time and money into a Ph.D.!

-You need to demonstrate that you have mastery of your subject area and the ability to explain things clearly to others. If you’ve ever tried to teach someone something that you’re shaky on, you’ll understand. You really need to understand your subject to be able to teach it well. One of my advisors once told me that teaching really prepared her for her defense.

-You get to know faculty, which is also useful, because again, the defense.

-For some colleges, it’s easier/cheaper to get the grad students to do it than the professors. It’s also a way for the grad students to be productive and at the same time earn money. Grad school ain’t cheap.

If you’re interested in being a therapist, you might want to get a Psy.D instead. It’s geared more toward teaching you skills you’d use in the field instead of preparing you for academia.

Edit: If you want to do research (especially your own research), someone needs to fund it. If you’re lucky, you might get someone from the military or interested corporations who will want you to do research for them on their topics. If you want to pick and choose what you research, you need a university, and that means you’re faculty, which means you’re going to teach. That’s how it works.

Some get ahead to prusing their goals earlier than others and well the PhD students had a handle on it first.

You don’t think a real professor doing real research has the time to teach Undergraduate Pyshc 101.

Also it’s good training to pit your witts against the snotty undergrads.

After a while they might pay you and they helps bide the time while you wait for them to review your dissertation.