Both aircraft are going the same way, both aircraft are cleaning up about the same altitude and accelerating at a somewhat similar rate, and both aircraft are accelerating to the same ultimate airspeed - 250 knots. Other than different climb rates that could prevent the trailing aircraft from seeing the preceeding aircraft over the nose (then you either see him or don't - it isn't about "refusing'), what is the issue?
Compare it to accepting visual sep on approach. In that case, both aircraft configure completely randomly and fly speeds that may or may not be the same. In addition, the crew is working to configure at the appropriate speeds without getting to slow for the current config (leaving few maneuvering options available to the trailing aircraft to maintain sep), they are working to align with the runway while maintaining terrain clearance, they are working to avoid wake turb issues, and they are running the Before Landing checklist.
Barring aircraft malfunction or weather (neither seem to have been mentioned), take-off is a much more benign environment relatively speaking and I see no issues with the aircrew agreeing to stay visual if it results in decreased spacing, decreased ground delays, and increased fuel savings.