There is definitely something to be gleaned by getting PT treatment but I'm not sure I would say it's the same as getting observation hours. Though, if there are places that accept hours receiving treatment as observation hours I would encourage you to use them, as I know how difficult it can be to slough through the required hours.
I've also experienced PT (for a bum ankle) and I know that during the process you should be focused on your own exercises and patient goals. I wasn't exactly paying attention to what was going on around me - more on listening to the therapist and trying to feel what movements hurt, what didn't, paying attention to how my body was responding and communicating this. Again, how can you really say you are shadowing someone when he/she goes around the corner to talk to or work with another patient and you are there doing your own exercises? I think it's smart to use these experiences as something to bring up during essays or interview processes where you can speak to how your relationship with your therapist inspired you or made you aware of the personal nature of PT care giving. I had personal conversations with my therapist and realized that was the type of relationship I wanted to have with my future patients.
Any idiot can stand around and watch a PT fill out paperwork.
Right, but this shouldn't be anyone's attitude coming into observations. You aren't there to watch someone fill out paperwork - you're there to gain an understanding of the profession, pick the therapist's brain, ask them tough questions - 'What frustrates you the most about your job?' etc, be as involved as you can possibly be in dealing with patients, speak with the patients while they are doing their exercise, demo exercises, ask the PT to quiz you on parts of the body. If you don't have time to amass a large quantity of hours, focus on the
quality of your hours. Get the minimum required hours and make sure you stand out as a really great observer who is absolutely passionate and curious about the profession. I didn't do a crazy amount of hours but got very involved in the process at a with a particular outpatient PT and he wrote me a really damn good recommendation. This will help your application more than just standing around for 300+ hours.