To sum up, managed descent, DES or VNAV, has limitations. It has been made with given parameters. And of cours it is as food as the lateral flight plan on which it is based. There is a great number of reasons to use V/S instead.
So it is very usual to use V/S or OP DES / FLCH tactically. It is simple, cheap and very effective and it doesn't take a PhD to use, anyway...