For GSM simply receiving everything on PCH does not buy you much, because even ignoring encryption (IIRC even PCH is encrypted) most of traffic there is simply

For GSM simply receiving everything on PCH does not buy you much, because even ignoring encryption (IIRC even PCH is encrypted) most of traffic there is simply RR sublayer signaling (RR PAGING REQUEST and RR IMMEDIATE ASSIGNMENT messages and nothing much else). What the linked presentation describes is somewhat obvious DoS attack on the infrastructure that depends on ability to cheaply inject larger number of SM-MT messages into the network which authors assume will exhaust the downlink capacity of the network. My assumption is that in such case the first resource that will actually get exhausted is uplink capacity of RACH as the paged MS will attempt to allocate SDCCH in order to complete the paging procedure. I also assume that any currently deployed implementation of GSM has reasonable enough QoS implementation that this cannot happen in practice (especially in relation SMS because various SMS-related accidental RR exhaustions were somewhat common in early implementations). By the way nothing prevents you from causing cell-level DoS of similar type by simply spamming the RACH (except the fact that your local equivalent of FCC will be very unhappy and will be knocking on your door in surprisingly short time).