Do you have to get one? No. But it doesn't come included in any cat-back system, so If you want the additional flow and sound a mid-pipe will provide, you'll have to spring for it separately.
The difference between an H and an X-pipe is how the crossover is designed. As their names state, so is the shape of their crossover. H-pipes tend to be a little louder and provide a little more low-end torque at the slight expense of top end and the opposite is true for an X-pipe. That being said, don't put a whole lot of stock into it either way, as the differences between the two as far as power is concerned is minimal. Just buy them for sound.