Full Idea
It is easier to imagine what it is like for a sentence to lack a truth value than what it is like for a sentence to be both truth and false. So I am grudgingly willing to entertain the possibility that certain sentences (like the Liar) lack a truth value.
Gist of Idea
It is easier to imagine truth-value gaps (for the Liar, say) than for truth-value gluts (both T and F)
Source
Leon Horsten (The Tarskian Turn [2011], 02.5)
Book Reference
Horsten,Leon: 'The Tarskian Turn' [MIT 2011], p.25
A Reaction
Fans of truth value gluts are dialethists like Graham Priest. I'm with Horsten on this one. But in what way can a sentence be meaningful if it lacks a truth-value? He mentions unfulfilled presuppositions and indicative conditionals as gappy.