The SPI was created for the Spartan-III program, the next generation of Spartans. The MJOLNIRs were hugely expensive to build and the IIIs were intended to be created in larger numbers and thus, more expendable than their predecessors. Makes a lot of sense, really. Sending a bunch of super-soldiers on near-suicide missions wearing hugely expensive powered armor is just the dumbest idea ever.
|Plus the helmet just looks cool.|
The reason why I like it is simply because it's realistic. It's camouflage system uses photo-reactive panels built into the surface of the armor to create a low tech (compared to the Covenant's) active camo system. In the real world, researchers are developing similar technology. It also isn't overpowered like the MJOLNIRs, which will literally crush the bones of any unenhanced human who tries to use it. If I were creating a futuristic military for a sci-fi story, this is the kind of body armor the soldiers would wear.
Pictures via Halopedia.