Just because someone doesn't ride the subway doesn't mean they don't benefit from it. More people on the subway means fewer cars on the road, cleaner air and water, less maintenance on roads, safer streets.
I live in Anne Arundel. Folks here are now footing the bill for WMATA thanks to the Maryland funding commitment. Metro is doing nothing to reduce congestion anywhere around here (there is no Metro station in the county).
People that would (without Metro) have no way of getting from MARC to their DC jobs would disagree with you that Metro it doesn't reduce congestion on 295 and other congested roads that pass through Anne Arundel.
2,000 would be 2%. That's in the range of a traffic change (1% - 5%) that would degrade the Level of Service of a highway from D (design goal) to E or F.
It doesn't take much of a reduction in traffic to significantly improve free-flow. Or, conversely, it doesn't take much an increase in numbers to make traffic suck more than it already does.