Not sure I agree, but will address soon.
I did quite well in statistics. Not tutor level and over six years ago, but quite well none the less. So, in a limited sample, say 4 of which over a large enough series of trials, only 66% or 2.5ish actually hit and about 2 wound. For each dice, if the invulnerable is a 4+, you have a 50-50 chance of passing. However, what are the odds that two consecutive rolls will both be higher than 4? I am not asking what the odds of a 4+ are because that is 50-50. I am asking what the odds of two consecutive 4+ are. So, the first roll does have a value.
http://arnoldkling.com/apstats/coins.html
What I am talking about is graphed as Pascal's Triangle in this link and fairly well explained.
In the above, I am assuming that the eldar get a 4+ invulnerable save. If they don't. Modify the above to whatever their actual save is.
A swarmlord only gets 4 attacks, of which only 2 will wound and those saves should be re-rolled. Unless, as someone mentioned, there is an FAQ stating otherwise. To do otherwise is a house rule.