Black Widow Vs. Valiant Vs. Indra

That right there is all the reason you need! 

Also as for the Indra, even though it's classified as a sniper, in practice it pretty much acts as a scoped automatic. So the comparisons to the BW aren't exactly fair. That being said, it can be put to great use if you do use it as an assault rifle. Pretty decent damage and good ammo count makes it a very viable option. 

The Valiant, while a great weapon, lacks the hard hitting power of the BW and loses out in comparison overall Imo. It is great for cleaning up mooks though.