(C) Circuits of 50 Volts or More.

The use of insulation that is white or gray or that has three continuous white or gray stripes for other than a grounded conductor for circuits of 50 volts or more shall be permitted only as in (1) and (2).

(1) If part of a cable assembly that has the insulation permanently reidentified to indicate its use as an ungrounded conductor by marking tape, painting, or other effective means at its termination and at each location where the conductor is visible and accessible. Identification shall encircle the insulation and shall be a color other than white, gray, or green. If used for single-pole, 3-way or 4-way switch loops, the reidentified conductor with white or gray insulation or three continuous white or gray stripes shall be used only for the supply to the switch, but not as a return conductor from the switch to the outlet.

So you see that they can be used as an ungrounded conductor as long as it is properly re-identified.

From the 2014 NEC Handbook-

Examples of applications where it might be necessary to re-identify and use a white or gray insulated conductor as an ungrounded conductor include water heaters, electric heat, motors, and switch loops. The insulation is re-identified to avoid confusing it with grounded conductors having white or gray insulation or neutral conductors at switch and outlet points and at other points in the wiring system where the conductors are accessible.

In previous editions, the Code permitted switch loops using a white insulated conductor to serve as an ungrounded conductor supplying the switch but not as the return ungrounded conductor to supply the lighting outlet. Prior to the 1999 NEC, re-identification of a white conductor used for this purpose was not required. However, electronic switching devices with small power supplies are available that can be installed at switch locations. These devices require a grounded conductor in order to power the internal components, and they help in clearly distinguishing a grounded conductor from ungrounded conductors, which aids in properly connecting such devices. Although in switch loops, the conductor with white or gray insulation is re-identified at all accessible locations to indicate that it is an ungrounded conductor, it is only permitted to supply the switch and cannot be the switched ungrounded conductor at the controlled outlet.

From New Hampshire document on common code violations-

“A conductor with white or gray insulation that is part of a cable assembly has been used to supply to a single pole switch as part of a single pole switch loop for a luminaire and it has not been reidentified by painting or other effective means (taping) at the point of supply or at the single pole switch.  This method of switching luminaires and receptacles has been a common application for many years and prior to 1999 did not require the white or gray conductor to be reidentified.  Therefore, this requirement is often overlooked.”

So in 1987 it states at its termination and each location where visible

From  1987 NEC

200-7. Use of White or Natural Gray Color. A Continuous white or natural gray covering on a conductor or a termination marking of white or natural gray color shall be used only for the grounded conductor.

Exception No. 1: An insulated conductor with a white or natural gray finish shall be permitted as an ungrounded conductor where permanently re-identified to indicate its use, by painting or other effective means at its termination, and at each location where the conductor is visible and accessible.

Exception No. 2: A cable containing an insulated conductor with a white or natural gray outer finish shall be permitted for single-pole, 3-way, or 4-way switch loops where the white or natural gray conductor is used for the supply to the switch, but not as a return conductor from the switch to the switched outlet. In these applications, re-identification of the white or natural gray conductor shall not be required.

In 1975 it states at its termination and and at each outlet where the conductor is visible and accessible.

From 1975 NEC

200-7. Use of White or Natural Gray Color. A Continuous white or natural gray covering on a conductor or a termination marking of white or natural gray color shall be used only for the grounded conductor.

Exception No. 1: An insulated conductor with a white or natural gray finish shall be permitted as an ungrounded conductor where permanently re-identified to indicate its use, by painting or other effective means at its termination, and at each outlet where the conductor is visible and accessible.

Exception No. 2: A cable containing an insulated conductor with a white or natural gray outer finish shall be permitted for single-pole, 3-way, or 4-way switch loops where the white or natural gray conductor is used for the supply to the switch, but not as a return conductor from the switch to the switched outlet. In these applications, re-identification of the white or natural gray conductor shall not be required.

In 1959 is says where visible and accessible

From 1959 NEC

200-7. Identified Conductors in Grounded Circuits Only. Conductors having white or natural gray covering shall not be used other than as conductors for which identification is required by Section 200-2, except under the following conditions, and then only where they are, in other respects, suitable for use as ungrounded conductors in a circuit:

Exception No. 1: Identified conductor, rendered permanently unidentified by painting or other effective means at each outlet where the conductors are visible and accessible, may be used as unidentified conductors.

Exception No. 2: Cable containing an identified conductor may be used for single-pole, three-way, or four-way switch loops where the connections are so made that the unidentified conductor is the return conductor from the switch to the outlet.

white or natural gray conductor is used for the supply to the switch, but not as a return conductor from the switch to the switched outlet. In these applications, re-identification of the white or natural gray conductor shall not be required.