I believe I fall into this category. Well, I do it because:
1) I'm by no means a competitive dancer, but I find having a dedicated partner advantageous when trying to progress to higher levels. I needed someone to grow with me. Switching partners every so often (as I have done in the past) has really not been helpful because I'm starting with a blank slate each time, and it takes time for a casual partner to reach my level.
You see, where I live, I have a hard time finding a partner at my level (very few people continue past beginner's classes). The only way to have a partner at the same level as me is to actually find someone who is rank beginner, but who is dedicated, and then take classes with them. When you cannot "find" a good partner, you have "grow" one, I suppose.
2) I'm introverted, so having a string of partners with whom I can relate superficially does not appeal to me. It wears me out emotionally. With one partner, I can get to know their personalities, their preferences etc. A partner isn't just a body to dance with -- they're real people too.
Yes, I realize I might pick up bad habits from dancing with the same person exclusively, but it's better than not having the chance to practice regularly with one person.
3) I'm pretty short (for a lead, 5" 4'). Fortunately, I found a girl who was 5" 1', so I clung to her, knowing that I'll never find a partner that short again. ;-) I know people say height differences don't matter -- well, that's not entirely true (especially if you're the lead and you're shorter). I agree that height differences per se aren't dealbreakers (because you can compensate for them through the use of proper technique), but you have to admit that certain ranges work better than others.
Also, my relationship with my DP is an exclusive one only because it's so hard for me to find another DP that's such a good fit for me. We have similar personalities, backgrounds, and body types -- we get along well. Otherwise she's fine with me partnering with other people.
My DP and I don't interact outside of dance at all, but as dance partners, we work well together and I really appreciate her. To me, it's a win-win arrangement.