ACKs are generally so small its irrelevant, unless your bandwidth is ridiculously asymmetric (less than a 23:1 ratio 64B ACK vs 1500B packet) which even really crappy cable providers don't do.
It's a very real problem for many if not most surviving DSL connections. Cable has gotten a bit better in recent years, but 10:1 is still about the best you can normally expect, and 20:1 or 25:1 is still common for some of the higher speed tiers on cable. So ACKs can realistically be expected to take up half of your upstream bandwidth on common connection types. That's far from irrelevant unless you never transmit anything other than ACKs and HTTP GETs.
