To be more precise, pretty much every economist you've ever heard of agrees with this statement, which includes a couple of key caveats:

Letting car services such as Uber or Lyft compete with taxi firms on equal footing regarding genuine safety and insurance requirements, but without restrictions on prices or routes, raises consumer welfare.

This comes from the IGM Economic Experts Panel, a recurring policy poll of several dozen well-known academic economists run by the IGM Forum at the University of Chicago's Booth School of Business. About 40 economists weighed in on the statement (and, because they're economists, they also couched their answers with a level of confidence from one to 10).

The strikingly unanimous results:

The taxi industry has argued quite the opposite: that consumers will lose out if cities open up the local transportation market to non-professional drivers who won't bother to serve low-income communities or disabled passengers.

So why do economists disagree? From the brief comments in the poll, here's Austan Goolsbee, a professor at the University of Chicago who was previously chairman of the Council of Economic Advisers (hyperlinks are mine):

yes. yes. a thousand times yes. Instead, try calling for a cab on Saturday night from the south side of Chicago and see what happens.

And his Chicago colleague Richard Thaler:

But Uber needs to be careful about surge pricing in emergencies, people care about fairness as much as efficiency.

And Oliver Hart at Harvard:

I don't see any externalities. According to standard economics, competition enhances welfare and I believe that would be true here.