How does one define a "full field of candidates" then? Labour and the Conservatives, yes, the Lib Dems maybe. But UKIP? Greens? Should the SNP have to stand in every seat? Sinn Fein and the UUP? The Monster Raving Loony Party?
A rule like this would make the General Election in Northern Ireland more interesting, that's for sure.
I see nothing wrong with the major parties choosing not to field a candidate in an election held in such circumstances.
I had more of an issue with people not standing against David Davis in his political stunt by-election in 2008.
If you voted for a fringe candidate because your preferred "main party" chose not to stand, then you're an idiot.
The SNP might do quite well in some northern English seats...! Just an aside.