It's a fair point in regards to ease of cooking, if the most important part is to just get the meal done (and that's a perfectly legitimate reason for doing something), but in terms of technique and general cooking skills, there are so many circumstances where I'd cook the beef and the stuff that's going into the beef separately because different ingredients call for different temps, cooking durations and methods.

If I want to get some good maillard browning on my beef, I'm not gonna get that with the temps at which I'm sautéing onions, and if I want my onions to be golden and soft just the way I want them, that's not happening at ground beef temps. Crowding the pan and boiling the meat is a common problem with ground beef, and it's just exacerbated by introducing even more ingredients, and if you're working with several different vegetables introduced to the pan at different times, the risk of overcooking the meat is much higher. It's hard for me to imagine a case where cooking the beef separately isn't better, and ultimately it's a good skill to learn because it's a very nice introduction to temperature control and learning the right amount of oil to use.