MS doesn't boost anything. It brings forth completely new abilities that are unique to you. For example:
Sasuke:
Blaze Release
Amatirasu
MS genjutsu
Susanoo
Susanoo covered in Amatirasu. Look at all the ninjutsu he got from just Ms itself.
Itachi:
Tsykiyomi
Amaturasu
Susanoo
And all those legendary weapons that he got separately.
Madara:
PF susanoo
MS genjutsu
and more to come
Shusui: Kotomatsku
kakshi: Kamui.
Point being made: Sharing an also offers a wide variety of ninjutsu power ups.
Don't forget the ability to copy ninjutsu as long as one has the chakra nature for it, making it more adapt than reningan in certain situations.