The weight of the larger Heiser tanks is not so much the problem, it is the bouyancy traits.
For example:
Heiser 140 - Weight = 63lbs, Bouyancy = -18.04 (empty) -28.4 (full)
Double steel 72's with manifold and bands - weight = 65 lbs, Bouyancy = -2 (empty) -12 (full)
The twin 72's hold 142 cu feet at 2475 psi, but are a couple pounds heavier than the Heiser 140. The big difference is that the Heiser tank is 16 pounds more negative than the twin 72's.
If you have to dive with a lot of lead, due to a very bouyant exposure suit, this may not be a bad thing as it would reduce your total weight (tanks and lead) by 18 lbs. But the tank weight is non ditchable and if it results in you being over weighted as well, it is a serious safety concern. At a minimum, redundant floatation becomes almost a neccesity.
So whether a Heiser tank would be a good choice or not would depend on your entire scuba system considered as a whole.
And another consideration is that even with an H or Y valve, you do not get quite the degree of redundancy that you would get with isolator manifolded doubles. So pound for pound most divers who really need that much gas are probably going to be better off with doubles.
