As for the ED, I would add one thing on top of what Magic says: since it slows down all ships to a minimum of 1/2, it can make your chaff slow enough to actually not get in range of enemy missiles quickly enough to draw fire from your main combat ships. This is because most players put basic Warp 6 engines (LH6 or FM) on them. I've seen that a couple of times in testbeds... big ships dying first, then chaff getting pointlessly massacred as they caught up to range.
Basically, when playing against a sufficiently evolved SD, you should plan on having ED in almost every battle - especially a defensive battle involving a SD starbase. It really lets the starbase fire two more free volleys at anything coming its way, With a fully decked-out Ultrastation, that amounts to almost 200 free missiles. I'd rather stare into their exhausts rather than their warheads.