The usefulness is dubious for most house cats. Most households don't have mouse infestations, and cats who go outside actively endanger wildlife (and yes, that is an issue for humans as well).

I'd say our cats are useful. These are their stats:

Number of birds killed by cats in my household over the last 5 years: 1
Number of birds killed by going full speed, beak first, into a closed window, then tried being passed off as a successful prey: 2 Number of mice killed: +500
Other wildlife known to have been affected: 1 mole

Can other wildlife have been affected? Possibly, but we're confident with regards to birds. And the only other weve seen cats go for are mice. Not even rats, I guess they're too big.