The problem stems from the fact that Metrolink tries to be a 'turn up and go' service, like the Tube, and so doesn't have a timetable (publically). Unfortunately, a 12/15 minute frequency just isn't enough to run in that way. For example, just missing a tram from the park and ride at Ladywell can add 50% onto the length of my journey from there into the city. In that time I can have driven the rest of the way into the city, parked, and be in my office. It's just not practical to use the Metrolink for time-sensitive journeys.
I'm not trying to do Metrolink down, in many ways it's fantastic. My issue is that with a bit of thought and investment in the right places it could be a thousand times better.