It could be that having a damaged Spiritweb allows greater connection to the goings on inside the Cognitive Realm, specifically the presence of Shards and, in Kelsier's case, Cognitive Shadows.
Remember, Kelsier was already able to communicate with madmen, as well as those who were spiked. He lost this power after he Ascended, but presumably regained the power of communication. So, the communication power of madmen was added to the power of the spikes, allowing Kelsier to physically manifest.