The logic up until now has been that the Dragon's Breath, combined with Crack Shot, was a superior choice to the Firepike because the rest of the squad could quite handily explode any tank without help from the Exarch, whereas a heavy flamer (in addition to being rare in the Eldar list) is particularly well-suited to cover the weaknesses of Fire Dragons.
In 5th ed, the firepike may get a little more attention as most tanks' only protection (other than AV) will come from cover saves (whether from terrain, smoke launchers, or flat-out movement), which the Exarch (and only the Exarch) can ignore using Crack Shot.