This is actually kind of an interesting question, because it ties into the ongoing controversy over what a clash between Greek hoplites actually looked like. The existence of the butt spike is in itself a strong argument against the traditional theory that hoplite combat was a literal shoving match, in which all ranks of the two formations pushed forward to physically overthrow their opponents. It is difficult to imagine that this wouldn't have led to massive "friendly fire", given that the men in the front ranks would have no freedom to manipulate their spears; the men behind them would have been forced onto the butt spikes in front of them.
Instead, we should probably imagine a more tentative sort of engagement, in which formations would draw apart after the initial clash (if neither side had already broken and fled by that point), and begin a prolonged phase of irregular "pulse" combat. Space would exist between the lines, and individuals or groups would dash forward periodically to get a few blows in. In such circumstances, only the spears of the men fighting would be lowered, and they would be sufficiently ahead of the rear ranks that there would be little danger of injury.
An alternative theory, recently launched by Christopher Matthew (A Storm of Spears: Understanding the Greek Hoplite at War (2012)), is that spears were actually held underarm in such a way that the butt spike only projected a short distance behind each warrior. By gripping the spear towards the back end, and pressing it into the armpit with a fully bent right arm, hoplites could effectively cover the back end of the spear with their own body. This grip would allow for maximal forward reach when the arm was extended. In this model it would actually be possible for hoplite ranks to be quite tightly packed together without causing casualties on their own side.
Contrary to how spearmen are commonly portrayed, spears were usually held from the back, instead of at the middle. (When thrown, they were held at the middle for balance, but for close combat, the spearman wants as much distance from the enemy as possible).
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Here's a followup question:
What were the main differences between the way Greek hoplites used spears, and the way Roman legions used them?
