Alright, here's my $.02 on this issue, and it happens to be something that I feel strongly about...
(Let me preface me by saying that this is only my opinion, which may not work for everyone but it works for me).
I do not play double bass and I have little to no desire to ever learn. That being said, I know HOW to play double bass, I just choose not to...and I haven't practiced it much, so I have very little left foot endurance anyway.
I learned double bass while attending the Atlanta Institute of Music primarily because it was required, but I also like to be able to teach it should I have a student request it (which I have had a couple of times).
The reason that I don't play double bass, and a reason that I haven't seen discussed in this thread, is that I fear that it will make me a lazy drummer. I don't play music that requires crazy blast beats, and I don't anticipate playing any in the foreseeable future, and aside from that, I see far too many drummer using a double pedal for things that could easily be accomplished with a single pedal. For instance, one of my favorite licks to play, either in a solo or as a fill, is RLRLFF (R-right hand, L-left hand, F-foot), played as sixteenth note triplets or thirty-second notes. Almost anyone with a double pedal would use it in this instance...but why? By using the heel-toe technique (as demonstrated by Steve Gadd in his "Up Close" instructional video), you can play that lick beautifully. And believe me, you don't have to be Steve Gadd to use that technique...if I can do it, then anyone
I say, learn to play consistent doubles with one pedal. I've yet to see anyone do strong triples or quads with one pedal, but those are usually overkill anyway. If you really, really want to use a double pedal, don't limit yourself to doubles (remember, do those with one foot...) or blast beats. Use it for something really creative...something like that wicked right hand-left foot-left hand-right foot thing that Gregg Bissonette ripped off from Vinne Colaiuta. (Shown in this clip at 1:33