Why would it make any difference what thickness the pipe is? I can understand how it's important that the conduit stays connected together, therefore necessitating glue. But, the way some larger runs of pvc go together, I doubt that it could come apart due to ground movement, or backfilling, or concrete pours (assuming it is staked down).
More to the point though, it would seem possible that clay (as well as concrete slurry) could seep into the conduit along with the ground water, and make it hard or impossible to pull the wire out later. Maybe this is why you have to glue it together?
