I usually reply to e-mails from students asking for PhD positions to let them know if I am interested or if they are competitive / up to standards for continuing with an application but this tends to be more on the admin / funding and suitability to my lab. Yesterday, I received one such PhD application and the project / research proposal was so outside the "state-of-the-art" (effectively the proposal was to research something that already exists as a commercial product) that I think I should also comment on that.
Should I let the student know and give him some feedback (which is going to be on the harsh side) about his proposal. Is it common or should I not bother at all? To be clear, I have no intention of considering the application.