The Boston Globe makes their case in an editorial:

There is no mourning period when it comes to ensuring adequate representation for Massachusetts residents in the US Senate. Important votes on health care and climate change draw near. Surely the governor can find a temporary replacement whose views are consistent with those of Kennedy and the Massachusetts voters who kept faith with him since 1962. Michael Dukakis, an elder statesman of Massachusetts politics, would be one good choice to uphold the Kennedy legacy for the few critical months before a special election. The former governor’s career speaks of a politician whose explicit promise not to seek the office could be trusted, even if no law can be cited to enforce the pledge.
There is limited time to make this change. US Senate rules require the closing of an office within 60 days of a vacancy. That means that Kennedy’s extraordinarily able staffers would be out the door precisely at a time when they will be needed most to provide guidance on health care reform and other major issues. A gubernatorial appointee, however, could assume the payroll of the staffers and benefit immediately from their expertise.

And points out the obvious:

Some state lawmakers fear they will look like hypocrites if they change the law to allow for such an appointment. In fact, they will. The shift in 2004 was indeed a naked effort to block former governor Romney from appointing a fellow Republican if Senator John Kerry were to win the presidency. Kennedy himself played a behind-the-scenes role in the political sleight of hand. A Globe editorial in March 2004 took Romney’s side and urged the legislative leadership “to scuttle this undeniably partisan bill.’’ The bill passed, and its dispiriting effects are now in full force. But sticking with a bad position for the sake of consistency or to save face would hardly serve the residents of Massachusetts.
Current House Speaker Robert DeLeo and Senate President Therese Murray should acknowledge that the Legislature was wrong five years ago and should take the necessary, if embarrassing, steps needed to assure representation for the voters of Massachusetts now. The position of each is evolving slowly. DeLeo contemplates a possible hearing on the matter in a few weeks’ time. Murray has said even less. The gap left by Kennedy’s death, however, is big enough for both to call their chambers back into formal session. Massachusetts lost a senator. It needs a new one, not a drawn-out discussion on succession practices.