No, you don't NEED one [discrete graphics], but Aperture never ran great on a MacBook. They lowered the minimum specs and eventually made it run on a MacBook, but it was never really designed to do so.
But hey, the loupe was slow even for some users with a Quad Core, so....
Just to clear this up (if it hasnt been already):
You need a Core Image compatible graphics card, which is one that supports Open GL Shader (GLSL) commands [Wikipedia:Core Image] (ie, Shader Model 2.0, or in the Windows world, DirectX 9). This includes pretty much all recent graphics chips: the Radeon 9000 cards, Radeon X*** cards, GeForce FX/6/7/8 cards, Quadro 4000 cards, recent Intel GMA chips, etc.
Despite that the Intel GMA 950 and GMA X3100 are integrated graphics chips and vastly underpowered (compared to discrete graphics), they still support the necessary shader instructions. Thus, there's no reason for them to not be on the list of supported graphics cards.
I kind of dont feel like they lowered the minimum specs for it to run on the MacBook. The rest of the MacBook hardware is more than enough to run Aperture (if my G4 can run it, an Intel Core processor can easily run it), and it has always had a compatible graphics card. I have to wonder if Aperture even pushes the graphics card that much beyond the fact that it needs Core Image capabilities (but Ive never used Aperture on a MacBook, so I wouldnt really know).