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My understanding Phil is that when you magnify 2x, you are therefore spreading the same amount of hard-won photons across a 2x by 2x, ie 4 times larger area, thus double the magnification, reduce the brightness by 4.
Therefore the only times maybe they are of benefit is when visually observing an object so bright that the loss of brightness still leaves it as an acceptable object (the moon, maybe 2-3 planets), or as Paul pointed out when you're capturing frame after frame after frame several hundred times, to then stack these frames together and rebuild that original intensity.
That's how I figure it anyway, I'm still prepared to have someone change my mind about Barlows as long as the proof of the pudding is to be found in the visual observing of that same pudding!
I WILL state for the record that the only Barlow that I 'own AND use' at all is the cheapest one - my 2" 2x GSO barlow, which when used with my cheap Andrews 80° 2" eyepiece changes it from a great 30mm wide angle view with terrible seagulling in the outer view field, to a 15mm wide field view with absolutely zero seagulling at all!
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