Well this is true, that 4% haste seems small when you look at it from that perspective but there are two points to consider. The first: it's not an extra third of your total mana, just a bit extra on the blue bar. That bit isn't so little, but it's not enough to save a fight and/or change a playstyle. Which leads into the second: again, if it's excess it's excess.Edit:
Personally, I'd take the trade of an extra third of my mana bar against 4% haste (~500 haste rating) any day of the week, being on the side of the mana bar.
I don't believe 4% haste to be significant enough to warrant such a loss.