It is no secret that there just aren't that many tenure jobs around, and with the adjunctification of higher education, the situation may become worse. Who knows? Maybe I am pessimistic, but I don't see things getting better at this point. In any case, if I were to earn a PhD, if I didn't get a tenure track job, it would be fine. One of the things that often comes up in teaching jobs is the whole tenure thing. From where I stand, tenure seems limiting to me. Right now, for example, I have seniority in my job which gives me some protection. If I were to apply elsewhere (or be encourage to apply elsewhere), I would probably lose my seniority, but I could balance that off with a higher paycheck (risk/reward). When looking at tenure track jobs, not only does it seem like will you be getting a lower paycheck, there is the whole publish or perish thing, which means you have to bust your rear to get certain deliverables out the door just so you can tick that little box off. I am a big fan of researching when you have the passion to do so, not the industrial model that seems to come with publish or perish.
Let's assume that you don't perish, what's the incentive to continue that intellectual production? In most places it's some sort of merit pay, but in all honesty, it seem that, most merit situations aren't that enticing to continue that intellectual production as it was in the pre-tenure years. Tenure, I feel, is also like a jail cell. If you fail to get tenure, you need to somehow mask it when applying to other schools. If you do get tenure, and you end up leaving, then there is either something wrong with you, or your former institution, in which case people are looking for gossip. Since it's not cool to gossip, you get dinged both for gossiping and not gossiping (don't people have anything better to do?). That said, remember that there just aren't that many tenure jobs out there to begin with!
Compare this to working as a professional in higher education, or even outside higher education. There is much more mobility in non-tenure positions. People can come and go as they please (provided they have jobs to come and go to), and it's just accepted that people will be changing jobs more frequently as compared to tenured faculty for a variety of reasons; some work related, and some not. Heck, you may just be bored with what you are doing and looking for a breath of fresh air. Either way you are moving. This isn't the case with tenured jobs.
So, in thinking about a potential six or seven year PhD journey, I am starting to wonder. If I am already in a learning community, and my colleagues (most of them PhDs) are interested in helping me develop and grow; and if I am already continuing my learning journey on my own with their guidance - what does a PhD program give me, other than the bragging rights and an edge over other adjuncts?