Piled high and Deep.

A PhD is useless in practice.  Only needed if you want to teach.

Please don't consider doing a PhD unless you've given it plenty of thought, and determined that it's the best and only way to meet your professional objectives.  This normally means full-time work in an academic setting.  (And even with that, there are LOTS of PhDs who can't find a job these days.)  Forewarned is forearmed.

And if you are serious about pursuing the doc, you should know what sub-field you're interested in: history?  theory?  behaviorial factors?  technology?  computation?  Each of these (or in some combination) is its own realm of study, with its own set of faculty trained to teach it.

"Please don't consider doing a PhD unless you've given it plenty of thought, and determined that it's the best and only way to meet your professional objectives."

Yes. It is true, you should probably only get a PhD if you are looking for some sort of connection with academia, specifically teaching. However, it will also help with publications, grant applications, travel fellowships, and your private practice notoriety. Plus there is not much downside to a PhD. Usually it is paid for, and they give you a stipend or work experience through teaching. So I disagree that you should only consider it if it is the best and only way. Knowledge is power. Consider it as a way, and if it will help. It will certainly give you maximum exposure to academia.

Don't get it if you just want a better chance for getting a job at a firm. Too much time commitment.

You can do a fair amount of teaching with only a masters and good connections-especially if you mainly want to just be involved in studios. A lot of my professional colleagues taught design studios with only their masters while maintaining full- or part-time employment at a firm. A PhD really is for the more committed professor track where research, publishing, and teaching are going to be your main focus.

Thanks a lot for the comments. Here's my concern: I don't have a degree in architecture but I do have a degree in design. I don't necessarily want to teach but work in the design and consulting field within architecture and planning. I am hoping this degree will make me somewhat desirable for a firm since I don't have an architectural background. I'm I just barking up the wrong tree? I am hoping this degree will showcase me as an expert, a researcher or an adequate consultant...is this a pipe dream?

Wrong tree, most definitely.  A master's degree is far better and more useful for your needs.  With rare exception, nobody in practice will care about the PhD (once you've spent years and years getting it).