I've never heard a genuine account of them being used as improvised weapons; rather I've only heard people say, "They used them as improvised weapons."

For the purposes of simple organization, they were used to denote what that soldier did in the army. For example, a ball instead of a spike denoted that the person was an artilleryman. There's a type of helmet called a czapka, which is actually Polish, but the German and Austria-hungarian versions of these helmets near many similarities to the Pickelhaube (the spiked helmet). The czapka denoted that the wearer was an Uhlan (a type of lancer cavalry). But like I said, the czapka and the Pickelhaube were not technically the same helmet, they just bore similarities. From a distance, it would be easy to confuse the two as the same type of helmet with different ornaments on top, which is why I include it.

There are other variations of the Pickelhaube, but those variations have little to do with the spike and more to do with material, length of the neck covering, etc., so I won't include them. It should also be noted that the spikes, balls, and squares could be removed to better conceal them from the enemy, or at least make them less easy to see.

Source: An Illustrated Encyclopedia of Uniforms of World War I by Jonathan North and consultant Jeremy Black