A current regulator, or driver, is required to ensure maximum power and good lifespan of the diode. As mentioned, batteries can have spikes and surges. The diode itself will undergo changes in its operating characteristics as it warms up. Anything that causes the diode to draw too much current will cause COD (catastrophic optical damage) to the diode, leaving you with an expensive LED. Hence the need for current regulation. Research the LM317 driver on this site (DDL or daedal driver) or buy a drlava or rckstar driver from this forum. Save your poor innocent diodes life, use a current regulator.
The pot doesn't REGULATE voltage, it basically just comes down to V=IR. With a steady state situation, or something not subject to transients in voltage or current like a lightbulb or good LEDs, then you can get away with not having a driver. But batteries are not steady-state, they have transients. And, laser diodes are EXTREMELY sensitive to transients and other such things. So a laser diode needs a driver to be safe.