Not sure about the logic of your comparison.
I mean, Widow Mines are 4x supply and much more cost as well, so I would hope they do more damage. Their splash is actually less, but you also have to factor in the units they are facing. I think it would be more interesting to look at using banelings vs marines, and using widow mines vs zerglings, or roaches, etc. Just comparing one stat isn't really going to give you a full picture because Starcraft is so asymmetric.
I think taking these things out of the vacuum, you can see that the real effect is slowing the enemy down, because they can't just waltz their army in with well placed widow mines or face death. However, the other races have the ability to do similar things, so saying that terran can now do this and that is bad doesn't seem compelling to me.