Electrical – Necessity of electrical boxes

There are only 2 correct and safe ways to add a 3-prong outlet where formerly there was only a 2-prong one*:

• EDIT: now allowed in Code since this answer was written: Retrofit just a ground wire, using proper wiring methods. There is no need for the ground wire to follow the same route as the power cable. Grounds can be shared: You don't need to homerun back to the panel if you can reach another grounded circuit with the same size or larger ground wire also coming out of that panel.
• Safe way #1: run a new cable with an appropriately sized ground all the way back to the breaker panel.
• Safe way #2: Use a GFCI outlet (with the "test" and "reset" buttons), which will cut off power to the outlet if it senses an imbalance of current. (EDIT: As @EricLippert correctly points out in the comments, you can also replace the circuit breaker with a GFCI breaker instead of using a GFCI outlet.)

There are many wrong ways to add a 3-prong outlet, most of which are dangerous and some of which are hard to spot or test for:

• Wrong way #1: Using a 2-prong to 3-prong adapter without properly grounding the tab (common but obvious).
• Wrong way #2: Replace the 2-prong outlet with a 3-prong one, but only hook up the black and white wires (this is hard to see unless you open up the outlet box, but an outlet tester with reveal the problem).
• Wrong way #3: Replace the 2-prong outlet with a 3-prong one and short together the neutral and ground wires (this is probably the sneakiest because it will test as OK with an outlet tester but is just as unsafe as #1 and #2).
• Wrong way #4: Replace the outlet with a 3-prong one and connect the ground to a random piece of metal (the outlet box, a pipe, metal cladding on an armored cable). This too may check out as OK with an outlet tester but is not safe.

*There are actually a few other ways to ground an outlet safely, e.g. with armored cable with a bonding strip or a grounded box plus an outlet that allows grounding through the strap. But those are both pretty uncommon.

Code is a guide for localities to establish rules. As such, if a locale adopts the code as is, it will be up to them to enforce their rules. Now we are down to the inspector's decision as to whether you comply.

Since the switched outlet is serving functionally as a luminaire, many inspectors would probably take the view that luminaire switches need a neutral (grounded) conductor. Usually when there are two rules and one is permissive and one is restrictive, the restrictive one will apply (unless the permissive explicitly waives the restrictive).

Further, there are practical reasons to include the neutral at a switch for an outlet. Advanced switches, such as remote control devices or a proximity sensor, usually need a neutral. Obviously, devices that would compromise the outlet if used for purposes other than luminaires should be avoided. You don't want a switched outlet with a dimmer to be accidentally used for a vacuum cleaner.

Bottom line, the local inspector will probably be the last word, but I would include a neutral.