Why only one active partition in MBR

Why only one active partition in MBR

I was reading about bootstrapping of operating systems and read in detail
about usage of MBR. But wherever I referred, they mentioned that there can
be only one active partition among four primary partitions.And when the
code contained in the boot sector of that active partition is
executed,user is given a menu for selecting one of the operating
systems(in case of multiboot).
I do have following questions regrading my above description:
1. What are disadvantages of having more than one active partition in MBR?
2. And why only four primary partition?(Is it because of the limitation of
size of MBR)
3. What is the use of primary partitions other than active partition?