If one were to assign all the resources for a task to its summary task, then, at the very least, one would have to hand calculate the work and determine which period of time (month, week & etc. in which to record the planned work for the resources. This defeats the purpose of using a scheduling tool. Why not use Project to do that (of course, some manual work assignments may be needed for updates to tasks)?
Further, as stated by Heather, all the planned work is hidden in the summary task, and that makes it difficult to track status of each task, and to perform resource planning. If tasks slipped or needed changes to work or duration, this would make the schedule a nightmare to maintain. There would be no way to accurately maintain a baseline or to manage a project accurately.
This would be the equivalent of removing all the the gauges from a vehicle’s dashboard and driving by feel. How could you tell if the engine were overheating, or speed?
Even one task hidden in the summary task defeats the purpose of using a scheduling tool. Yes, it’s done, no, it’s not good practice.