You may or may not gain significant benefit from a separate dac. Use the cd player without a separate dac for a while, see how you like it, and perhaps try to obtain and evaluate a separate dac on a no-obligation trial basis.
The possible benefits of a separate dac include:
-- It MIGHT provide better sound quality than the built-in one, depending on their respective quality and on system synergy.
-- The ability to use it with other digital sources, some dac's providing usb inputs which can be fed from computers, for instance.
-- If the laser assembly or some other part in the transport section of the cd player were to fail, you would just have to purchase a new transport (or a new but inexpensive player to use as a transport), instead of replacing a presumably more expensive self-contained player.
A downside of having a separate dac is the possibility of jitter being introduced in the digital signal at the interface between the two units. Some dac's have sophisticated jitter rejection provisions, and some don't. If you choose one that does (such as the Benchmark units), you would likely get excellent results even with a very inexpensive player or transport.
Hope that helps,
-- Al