You need to consider what your PhD will give you beyond a few letters after your name.
My story is that I spent my early career as a Research Assistant but could see that a continued career in Academia would require a PhD. On the other hand, a move into industry would require a more marketable skill than theoretical Ecology and the ability to Find Stuff Out. So, I designed my own PhD program that could not be completed without training in and heavy use of GIS (partly to provide a logical career progression from Ecology and partly because I saw it was a growing field as well as satisfying my techie talents). The University accepted my hypothesis and packed me off at their expense to an ESRI boot camp to learn ArcInfo 6.x (happy days!). The combination of my PhD and the GIS skills got me my next job as a Consultant in the commercial arm of a government-funded academic research institute. Since then I have moved out of academia altogether and my PhD has either been irrelevant of itself or has been a minor positive asset usually only for the kudos such a qualification provides in consultancy.
So, in my experience, a PhD is a minor benefit as a Consultant, ticking the credibility box. It has never really been a hindrance, but the most important thing I got out of the PhD was not the letters after my name but the training and demonstrable experience, which has allowed me to make a career in a growing and multi-faceted industry.
However you need to think carefully before undertaking one as opposed to, say, a taught Masters or other course of study which can be completed in a much shorter space of time and arguably gives a greater breadth of training. A PhD is highly specialised and it is a long and lonely furrow to plough, especially part-time. I did do mine part-time in only three-and-a-half years, but then I had been a professional researcher for many years before that, meaning that I wasn't learning how to do research as part of the PhD - which would be the case for most people.
If your aim in further study is to enhance/gain skills in GIS, I believe that you will get a quicker return on your investment of time and money in a taught Masters (say), plus some small additional academic kudos. If you think your career may ultimately take you into Academia as a Research Fellow, Reader or Lecturer, then I would recommend the PhD.