There is little demand for longer orbital journeys, e.g. Bolton to Rochdale or Bury to Oldham, and there are adequate bus services for the moderate ridership between adjacent towns. Metrolink's primary function is to provide high capacity electrified rail transport to/from the city centre, and as there is already a good fast train service from Bolton town centre to all stations in central Manchester, there is no need for Metrolink to serve Bolton town centre.
The bus services in the area are not adequate as shown by the high levels of motor traffic from Bolton to Bury on the A58 and Bolton to Rochdale/Heywood via the A666/M61/M60/M62/A627(M) route and vice versa; in my view, the bus services are not adequate for their purpose because of the lengthly journey times between the 3 centres (Bolton, Bury and Rochdale).
Much of this motor traffic would be attracted away from the road by a Metrolink service as shown by the heavy use of Park and Ride car parks at Radcliffe and Whitefield.
The primary purpose of Metrolink is of, yes, a radial tram network to provide interurban transport between central Manchester and the other big centres in GM, however, it provides an ancillary local transport function particularly in the Wythenshawe area of Manchester and within the boroughs of Trafford, Bury, Salford, Oldham and Rochdale.
A reopened Bolton to Bury/Rochdale line would also provide direct connections from Little/Darcy Lever to Bury/Radcliffe, a currently badly served flow.
Bolton to Rochdale would also provide for flows from West Yorks (Calder Valley line)/Oldham/the Rossendale Valley/most of East Lancs to Bolton/Bury without going into central GM, which would save a lot of money and time for many commuters and take a decent amount of traffic off the M62/M66/M65.