It is more detrimental than most believe it to be. I'm well aware of the poor habits of many, that does not make it anywhere near optimal.
Most do things because "I heard it from...", some do it because "I read something that said...", and few do things due to; 1. A greater understanding of the science, 2. Their own empirical knowledge, and 3. The application of the two.
Ask the vast majority of those growers why they expose their veg'ing plants to 24hrs of light. Not one will be able to produce an answer that disproves any of the known science. The common answer will be "more light is better", some may say "C3 plants have light dependent and light independent phases, so dark is not necessary in veg". The former is just misinformed, the latter incorrect. Though the light independent phase does not require dark (for SOME, not ALL metabolic processes), it is indeed compromised in the presence of light. There are processes that do not take place in the presence of adequate light.
Edit- "light dependent and.."