There's nothing forbidden about it but airmanship and common sense should preside here - if you are climbing at m0.8 in managed climb and get a negative v/s then go to open climb with selected speed and notch off a couple of decimals of mach. If need be take it back to green dot. Then, if something distracts you, you aren't going to DIE. Yes the protections are good, but bad habits form and a later conversion onto another type without protections could result in a nasty accident. You get the same result at the end of the day, but through a safer way of achieving it.
Another example would be glideslope from above, I can't stand guys who select 100 feet.
