After extended durations in combat, they become more of a liability.
Indeed.
I'd wager that I've killed more players who couldn't get away quick enough because they were weighted down with SCBs than I've fought players who were saved by their SCBs.
I've also lost my shields, mid SCB use, and dropped the shields of others even when they popped a large SCB at just the right time. They don't refill shields instantly, and the total charge is dependent on size and quality. You also cannot activate a second one from the same bank until the first one is done charging.
Oh. I was not aware of that. That is counterintuitive.
Not really, when you think about it.
Also, sometimes you want to fire more than one at a time.
Two smaller D class SCBs will weigh much less than a larger, higher class one, but used together may restore similar amounts as the heavier unit. If you are more concerned with surviving an encounter of limited duration, being lighter, while still getting the same shield strength restored per use, may well be more important than having extra uses.
With my viper I run multiple banks at least 2 and sometimes a 3rd depending on the target or conflict I am anticipating...
I can't ever see a reason for this.
That amount of weight is suicidal for PvP on the Viper, and no NPC ship should absolutely require any SCB usage at all.