In addition, when a processor is idle it drains any DPCs that are in its queue. Remember that the default behavior of DPCs if you use them without targetting or setting their importance, is that they are of Medium importance and will not be targeted. Like I said in the introduction, you may never have a need to call upon the advanced capabilities of DPCs. Only one driver shipped with NT, NDIS, uses DPC targeting and priorities. NDIS uses low importance DPCs that are targeted at a particular processor for its DPC processing.
Many devices require timely response by their interrupt handlers.
Like most driver-related issues, it really depends on the other drivers that are in the system.