Perception of Planners related to land development (gentrification) is that Rail is Good and Buses are Bad for many of the reasons discussed above (especially: it will still be there tomorrow, and it's "cute"). Perception of Planners and many advocacy groups related to social services (anti-gentrification) is that Buses are Good and Rail is Bad because buses are much less expensive to buy and run so you can run more to more places (though not as many seats to one place). Of course, in the modern era, rail also needs lots of Federal money to build it (cost is immense), which brings in a lot of extra requirements, so it's a balancing act. Then there's the cost of operation, which is still far higher for rail than for buses (buses don't have to pay for power systems and track/right of way - the road's on somebody else's budget and if they burn CNG the fuel cost is fairly low), balanced by the ability to adjust capacity per driver by running longer trains (one driver) instead of more buses (at a driver per), but the operating cost balance is still in favor of buses on all but the most heavily used lines.

My impression is that places like Sacramento, SF Bay Area, and parts of LA/San Diego are where some rail transit makes sense in CA, and strictly speaking even Sacramento is a stretch. I suspect that San Diego's system is a little overbuilt but much of it was cheap enough that it works. Essentially, you need a corridor where buses just can't handle the load - where you can't run enough of them at rush hour. Not many of those around. Sacramento's light rail is run more like a trunk-line bus than a train (26 stops in 23 miles?), but with feeder bus lines adjusted to make use of that it works OK. Just think of "real" transit (buses and light rail especially) as a social service, not transportation, and you get what market they serve - rail can help with that but buses are more effective in most places.

My opinion of most modern "streetcar" projects is that they're amusement rides in connection with a development (or redevelopment) project that can benefit from "cute" in-street service that by its presence messes up traffic to the point where people won't drive but still want to be there - and that can pay for the line both to build and to operate through taxes and fees. That's what the Portland Streetcar is. Possibly, also the SLUT. SF is a special case. Usually, more buses would be more effective for raw transportation purposes than a streetcar and much cheaper and cover a wider service area.