Should you get an external flash? In general, IMHO, if you can... Do... What I didn't like about the flash that you listed was that you don't have the ability to bounce the light. Being able to bounce the light will give you more lighting options.

For macro photography if you have enough abient light, an external flash isn't a necessity. If you bought a ring flash, however (something like this), it would free you up to be able to just squat infront \of anything and take a shot, rather than trying to figure out how to get enough light to the subject.