The alignment aspect is something I've read about and can see that it makes sense. Last summer I was on a week long tour and meet a guy who was touring and told me about his bad experiences with his bikes external bb. If I recall he had had them changed four times, and each time not losing more than something like 2 or 3000kms, at different shops. It didn't make any sense to me, but the possibility of poor facing and alignment could very well been the factor here, he was a tall guy who looked strong so this could have been a factor that played a part.
I will very much keep this in mind and make sure proper facing is done it I end up going with external.

As you say about it being lighter--I'm glad to hear these mentioned lifespans that are good. As for the weight, with fully loaded touring, I learned a long time ago to be careful of total weight, and so would welcome a reasonably priced crankset like that deore trekking one that will be lighter than square taper.