In a much-read post composed over the last week, I pointed out that replacing buses with streetcars is not, in itself, a mobility improvement. I said several times that this wasn't the only criterion for transit investments, but if you're going to call something a transit project, you might expect someone to ask if it improves transit mobility.
I think you are making a mistake in looking solely at mobility rather than access. A huge benefit (perhaps the largest benefit) of streetcar is the 'trip not taken'. By fostering a more compact form of development streetcar allows people to meet their daily needs with fewer trips or shorter trips (and shorter trips are less likely to be auto-oriented).