I think “you should never ___” arguments are a bit silly. It all depends on your personal goals, play style, personality etc. I agree that most vets would probably advise against spending CP on covers, but whether or not that works for you is up to you.
I personally spent most of my CP on covers when I was at the 2-3 stage and don’t regret it. It allowed me to get six championed 3*s within about two months of playing (though yes, I also spent money and did other things you’re not “supposed” to do like spending ISO for standard tokens).
Sure, if I’d spent my CP on legendaries or hoarded them, it would make the 4* transition easier, but it also would have taken a lot longer to get there. If you have the patience, it’s probably better to wait, but I don’t. And it’s true that you’ll eventually get the 3* covers if you wait long enough, it’s also true that you’ll get pretty much anything if you wait long enough. You will get every 4* champed without spending any CP if you wait long enough, but it will be a VERY long wait.
If getting to the 3* stage faster doesn’t matter to you, then you’re probably better off not spending CP on 3* covers, but the reason did was that I just didn’t find the 2-3* stage of the game very enjoyable. My options were too limited, and I found the 2s kind of boring to play. Once I got some 3s, that changed. Having them also allowed me to earn more 3* covers (by finishing in the top 50 in PvP etc.). It probably would have taken me at least another month or two (or three) to reach that stage if I’d spent CP on 4/5*s.
Plus, at the time, I didn’t have the roster space to have a lot of 4s so I ended up either having to sell them off or having my roster cluttered with unuseable 4s. I also wasn’t ready for 5s and sold off the few I got to avoid scaling (another thing you’re never supposed to do). Yes, one obvious solution is to hoard your CP until you’re ready for 5s (or at least 4*s) but I don’t have the personality to hoard CP for months (kudos to those who do - it would drive me insane).
Currently (day 99) I have all the 3s rostered [18 champed] and about half the 4s (none useable) and find the game MUCH more enjoyable, as I am able to use several different teams (instead of the same old 2* teams) with characters that have a much wider variety of skills (I suspect that the increase in enjoyment level at the 4* stage won’t be as great, but maybe I’m wrong) and be more competitive in more events AND spend less time grinding (with Thanos champed, doing seed teams in LRs takes about five minutes).