I'll point everyone to a good post by Jonathan Chait, who draws a distinction between pure posturing on the debt ceiling (as in the Democrats who voted against raising it in 2006), and "trying to extract substantive policy concessions."
I'd go a bit farther than that. First of all, it's worth mentioning that way back in 2006, long, long, ago, we still didn't have a 60 vote Senate: the debt ceiling increase passed by a 52-48 vote, with no cloture vote at all because the Democrats didn't filibuster it. As far as I can tell from a quick search of the reporting back then, the Democrats did threaten to attach amendments (and wound up forcing at least one recorded vote), but they didn't use it as leverage (by filibustering or threatening to filibuster) to, say, force a withdrawal from Iraq.
Now, in fact, I don't know that using the threat of default to win policy victories is irresponsible. Even bluffing that you're going to destroy the country if you don't get what you want...I don't know that I'd say that would necessarily be irresponsible. Actually going through with it, though: yeah, that would be about as bad as it gets. So I'd make a distinction not just between pure posturing and terrible behavior, but between pure posturing, responsible negotiations, and irresponsible negotiations. And I'll note that we probably can't guess which one is going on until the end of the game.