I don't have a specific reason why Pan Am or other entity might want double stack clearance on the Worcester Main but as long as public money is to be spent to acquire and upgrade the track, why not use that opportunity to raise clearances (again, I don't know if that is easy or hard to do on the Worcester Main as I don't know what the obstructions are)? But if this line were cleared to plate H, at some point in the future both 20' 2" high" AutoMax auto carriers could be forwarded to Ayer (perhaps the formerly CSX-leased auto transload facility might be re-opened?) as well as double stacked containers interchanged with CSX (or perhaps a successor owner of the B&A main). And obviously Pan Am Southern operation/ownership, if it continues as is, would have to evolve to accommodate this. My guess is clearing the Worcester Main using Massachusetts money is more likely than seeing anyone (eg, Massachusetts, NS, PAR) pony up the TBD but large amount of $$ necessary to raise Hoosac Tunnel clearances.
As a somewhat related example, when MassDOT acquired the Framingham Secondary from CSX (a line that has little to do with MBTA commuter rail service) as part of CSX double stack clearance project on the B&A, MassDOT dropped new rail and began upgrading the track on this branch line that sees only a pair of daily CSX freights whose consists continue to shrink.