That's David Alpert's frame in a piece in the Atlantic Citylab today (links added):

Jaffe, Walker, Yglesias, and Capps have no duty to support Team Transit [sic!] no matter what. They should speak their minds. And anyone who supports mass transit expansion should want it to be as close to perfect as possible. I worry about streetcar criticism that states that a streetcar without every desirable feature is worse than nothing. But streetcars also have another set of opponents: Those who simply don’t want to fund any transit at all, regardless of its specifics. They seize on any flaw to stop projects that might change their street or interfere with their driving. So I worry about the effects of this latest trend in streetcar criticism. While streetcar projects can and should be better, many of these articles go further and either imply or outright state that a streetcar without every desirable feature is worse than nothing. That’s not right. Perfect transit is absolutely a goal, but the perfect must not be the enemy of the good. There are plenty of reasons why a streetcar might be worth supporting, even if it isn't as long, frequent, or speedy as we might like.

I have spent my whole career helping people value what's really good-but-not-perfect in transit choices. Our difference is that in Alpert's framing of the question, the fundamental good to be defended at all costs is the streetcar technology, while to me the fundamental good is the liberty of large numbers of human beings, and their access to both happiness and economic opportunity.

Let us take Alpert's perfect-vs-good frame and deploy it differently. Many earnest American leaders visit places like Bordeaux and Strasbourg and agree their cities should look just like that. This looks perfect to them, but they realize they'll have to start with something that's good-but-not-perfect, an imperfect good.

Well, which "good" element should we start with? In Bordeaux and Strasbourg, the streetcar (never mixed with traffic) is a result rather than a cause of a whole bunch of other things: policies that limit car access, for example, so that transit of any mode can run reliably and so that it delivers people into a rich pedestrian space. The Bordeaux and Strasbourg streetcars also began with the "imperfect good" of bus services, which were used to build robust lines with actual existing markets that would support the future rail service.

Why should the "imperfect good that we start with" be the streetcar instead of a really liberating transit system run, for now, by buses? Why must we start with a hunk of decontextualized technology rather than our liberty and opportunity to go where we want to go?

Alpert goes on to make other points about why "imperfect but good" streetcars are worth supporting:

Imperfect transit can still be good for cities. Millennials, empty nesters, and others want walkable, livable urban places. Unfortunately, there aren’t enough of those in the United States, which is why they’re increasingly expensive. There are plenty of places on the edges of cities that could become more walkable, more urban, and have more of a sense of place. To do that, they need better transit, more amenities, and more residents—which generally means more density. When such a place achieves greater walkability and urbanization, the factors making it so strengthen over time. … It’s a momentum game, and even an expensive, sub-optimal transit solution—such as a less-frequent streetcar with no dedicated lane —can push the cycle in the right direction.

The sheer abundance of places that need to be made more walkable is actually the strongest argument against the streetcars-in-traffic campaign. In transit, if it doesn't scale, it doesn't matter. Streetcars-in-traffic have helped enrich a few superdense districts, but they are far too slow, unreliable, and expensive to scale to the size of our urban mobility problem — at least not as long as they remain stuck in traffic. (Once they get out of traffic, they are essentially light rail.) Nor are streetcars remotely necessary for the development of walkable, urban places.

If you want to see how a city massively expands the usefulness of transit, and thus the potential for transit-oriented lives, look to what is happening in Houston. Massive, scalable, high-frequency bus grids that are useful for getting all over the city, and that can be created now.

An imperfect streetcar might be all your city can afford—for now. … Your city can make it better later, and may even plan to.

The frame here is: "The streetcar technology is the essential good, and people's ability to access their entire city is a nice-to-have that we hope to add in the future."

But even if you accept that frame, what's the track record of claims that modern streetcars-in-traffic, first built in compromized ways, have led to later efforts to improve them? Perhaps you should study Portland, which has been living with this product for longer than any other US city.

The streetcar has been extended up to the limits of usefulness for such a slow-by-design service (about 3.5 miles). But there are no serious proposals for taking cars out of its lanes for enough distance to matter, nor is there much energy behind extensions. Why?

In Portland, support for streetcar spending has collapsed. A recent Bureau of Transportation poll found that only 38% of Portland residents would assign a more-than-neutral priority to further expansions of the streetcar. The same number for more frequent bus service is 67%. (Light rail, in exclusive lanes by definition, is at 59%)

The Portland Streetcar has taught Portland residents a lot about what's really matters as you define an "imperfect good." Listen to what they've learned: Frequent, useful, reliable transit — using tools that scale to the scale of the whole city — is the "imperfect good" that matters.