Try this analysis of various coasting or non-coastion options.
1. Normal driving In gear, clutch up, right foot on gas or brake. We have instant control of either acceleration or braking, and almost instant control of the reverse effect by just moving our foot accross. This is good.
2. In gear, clutch down, foot on brake Instant control of brake, but to get any power down we have to move both feet and get the clutch engaged, which takes time, and can destabilise the car if done roughly. Also, without the engine engaged, it is harder to know which gear you should be in, or what revs you need to match in. Also possibly reduced sense of speed of the vehicle. This is less good than 1.
3. In gear, clutch down, foot not on brake All the disadvantages of
2, plus you don't have any immediate control of your speed, in either direction. You are at the mercy of gravity etc. This is generally bad.
4. No gear engaged As for 3 but now you have even more work to do before you can get moving. Bad.
Whether any of this becomes an actual problem is down to circumstances. The rules do start to break down at low speeds. I will, sometimes, coast in neutral down a hill in crawling traffic if my speed is not getting above 10 mph, as this avoids a lot of unnecessary pumping of the clutch. The time to really avoid coasting is going into hazards, as this is where you especially need full control, and coasting here tends to result in getting your feet tied in knots when you decide to 'go'!