On a pedantic note people mean "plurality" and not "majority". A majority party would be definition be able to form a government on its own.
I think it depends rather on how close the top two parties are. In 2010 for example the gap between Labour and the Conservatives was something like seven percentage points or over two million votes, in which case a Lab-Lib coalition even if it had been possible at Parliament would have been seriously pushing it for legitimacy. Although Cameron didn't win in 2010, Labour certainly lost and badly.
On the other hand 2015 is set to be a closer race, and if the gap between first and second is very close (very possibly less than 2% overall) then I don't agree that the second party should be out of the running. Especially when it will be equally close in terms of seats.
The Lib Dems may yet have a role in the next government. A somewhat interesting question is whether their presence in a coalition is more legitimate than the SNP's. I would say yes, because the SNP are confined to only a part of the country and the Lib Dems receive support nationwide. And, while NJ may be technically correct, this is a vote that in practice is far more about who is running the country. The parties involved in that most directly ought to have at least a reasonable level of support across the country, no?