Though I agree that selecting flaps while on the rollout is a less than ideal action, some flight manuals do specify it, in which case is is worthy of consideration, if pilot skill and workload permit.
you sure as hell aren't getting back out of that field again anyway since TO requires a lot more field in SEPs than landing.
There are a number of types (Cessna 182RG, for example) where this is not so much the case. Before I lengthed my home runway from 1700 feet to 2100, getting the 182RG in was never pleasant, but it would always jump out. Similarly the taildragger amphibian I fly here now, uses much more getting in than out.
Generally, for lower powered singles, I would agree with the assumption of you remark though...