Several of you mentioned learning vulnerability and trust from BDSM. To me, this seems akin to mood-altering prescription drugs (bear with me) -- not the kind meant to correct an imbalance, but the kind meant to give a temporary mood uplift so that one has more strength and energy to learn/relate in ways that would otherwise be out of reach (or to temporarily block pain so that one can participate in physical therapy). I think they can be a great tool -- but I think that if one does not have the goal to eventually come off of them and develop the ability to learn/relate without them, then they become a hindrance. And like prescription drugs, BDSM can be used with a purpose or used for the high with no intention of gaining any other benefit. (those who commented did not seem to be in this camp at all, but the majority of those I have come across used BDSM as some use recreational drugs -- that I am 100% against)
On bondage: to me, it is the presence of bonds or use of force/control that creates a power imbalance -- even if they are chosen beforehand, even if one can say 'stop' whenever one likes. To trust someone not to abuse their power can indeed build trust, but that same building of trust can occur by both partners being vulnerable and thus giving the other person power (while retaining their own). If two people each have a weapon and no shield, both are vulnerable and both are powerful.
I feel that it can be useful, rarely, to lend your self-responsibility to someone else -- but I think that is not positive if it is continuous or the only way you can relate, and it is not necessary to give someone physical control of you in order to lend them your self-responsibility. For example, if x person collapses with grief and y person bathes, feeds, and takes care of them, that can be a wonderful help and bonding experience -- but if it becomes a habit it is irresponsible of x person and debilitating to y person. In the same way, I think it can sometimes be very healing to only receive and not give, but only rarely, never as a habit. And I think that it is a very important thing to learn how to only-receive without external restraints.
I think the core might be my belief that it is essential to have vulnerability/trust in the absence of power difference, and that it is important that both people learn vulnerability/trust equally (I think that if only one person is submissive as habit, the relating-style is not equal).
Ultimately, we live in a world of power difference/inequality. We are all very good at relating this way because we have no choice; it is all we are taught. Perhaps we are only good at controlling, or only good at being controlled, but none of us are taught how to have a power balance that gives equality (which you cannot learn by practicing the opposite of your habit!). And I think that by relating sexually exclusively with BDSM -- or with the dominant/submissive paradigm that is the current ideal of sex -- a person misses out on that difficult but amazing chance to learn a way to perfectly balance giving/receiving, leading/following, initiating/responding (within one experience and including both people)*. Theoretically one could MAYBE do both kinds, but it seems to me that the mindset necessary to participate in the latter kind of sex eliminates the inclination to participate in the former kind.
To further clarify on my boundaries with friends: I can still relate to and be friends with those who practice it sexually with positive intentions; if your BDSM exists only in sex then it's not an issue for me. If you do practice BDSM as your only style of sex I will think that you, like most people, are missing out on something important. I think you should know how I feel, so if it is a boundary for you to share your life with people who consider BDSM to be a way of relating equally, you can make an informed choice. I cannot relate at all with a person who chooses to let someone else decide zir behavior outside of the bedroom (or vice versa), so if that were the case I would not be friends (and that is not the case with anyone on my flist).
* would anyone be interested in a post on this sort of sex (which I refer to as 'equalist' sex)? I kinda feel like I expressed a lot of the underlying concept in these last two posts but if anyone is interested in reading still more I might make a post specifically on that