Just a couple comments. I don't think it is unusual to see some "mechanical joints" in piping systems "above grade", but use of plastic pipe in general in at least some above grade environments may well be another subject/and with other concerns (for whatever it is worth I sort of understand the problem as I don’t think there is even any mention of aboveground or above grade or flanged joints etc. in the “C900” you refer to, and the scope of this standard at least at one time specifically used the word “underground”).
With regard however to the basic piping question of a direct substitution of a mechanical joint vs a flanged joint (the latter with innate bolt “restraint”), there could be concerns as well depending on the specific design of restraint to be used to keep the plastic pipe end from pulling or deflecting out of the socket.
Also, (as I think at least intimated in bimr and SteveWag's good responses) a standardized mechanical joint for a fitting or valve has a socket depth or "stab length", wherein the pipe typically penetrates some distance up into the socket (I think up to about 2-1/2" deep on smaller pipe sizes, and deeper on larger). Unless the fitting is a mechanical joint sleeve, the pipe can therefore not be as easily lifted or withdrawn laterally from a line, even after complete unbolting of the joint, as can a flanged or grooved coupling pipe , say in piping/valving etc. replacement, re-routing or maintenance operations etc. e.g. as by very slight spreading apart of the flanges etc.) In other words the pipe end up in a mechanical joint can hang up on the penetrated socket end. This behavior may or may not be a future problem in the specific piping configuration/application, with sometimes other flexibility.
I will note also there are other sorts of devices available in the general piping field including e.g. "flanged adaptors", some of which offer a basic field-adaptability (cut to suit), a mechanical sort of sealing mechanism not unlike mechanical joint on one end with a flange connection on the other, and some even offer separate restraint features as well as more easy lateral pipe withdrawal features. Again however you might want to contact the manufacturers involved to make sure they are happy with use of these devices/restraints etc. with the specific above grade plastic piping application.
