Despite the headlines, what Jim "I'm Secretary of State for a part of the UK that has a devolved assembly so my name has to be" Murphy actually said was;
"I think it would be a real benefit to Labour's campaign if the prime minister can attend, so we'll be talking about that... But I think most people in Glenrothes in truth would rather he was certain he was doing everything he could in the economic crisis. If he is able to do that and come to Glenrothes, I think he would be a great boost to our campaign, so I think it makes sense."
Or in other words, the Prime Minister would like to talk up how much he wants to be involved and how positive it would be for the campaign, but in reality his appearance would go down like an iron airship so he's going to set Cameron up for the fall when he has another go at him for not going to a by-election and he can pivot to looking strong and responsible on the economy.
It's all very well trying to set a trap for your opponents, but erecting a ten foot neon sign above it and standing next to it waving your arms isn't usually the best way to make it work.