Only if all three phases go to windings of the same motor would it be safe to assume that they will stay in balance. In that scenario, if all three phases are supplied balanced power, the current in the neutral wire will be zero, and the voltage would be zero even if the wire were disconnected. If the supply becomes unbalanced, however, the neutral wire may try to play a more active role, for better or for worse.
There is at least one case where the neutral wire should not be connected to the supply. In some installations, three-phase motors are required but three-phase power is not available. It is possible to start a three-phase motor by connecting two of the legs to 240VAC, and connecting the third to a capacitor box. A motor driven that way will not be able to supply any significant torque until it up to speed, but once it is spinning, the third leg will generate voltage which is in the proper phase, relative to the other two, to drive a three-phase motor. One shouldn't try to draw too much current from this third phase, but it may be used to start a larger motor (again, one shouldn't demand torque from that motor until it is up to speed). Note that in such arrangements, the neutral wire of the assembly will not stay at ground potential, and must be allowed to safely "float" or else bad things will happen.