I think there's a difference between a "guilty pleasure" and a "noble failure". The latter is serious (even if comic) in intention and skillful in execution, but failed to generate sufficient popular interest to achieve commercial success. Most of Sondheim's musicals and BRIDGES OF MADISON COUNTY are, IMO, noble failures.
A guilty pleasure, on the hand and whatever its commercial success, is a piece that doesn't aim very high in the first place, but offers some sort of odd pleasure nonetheless. For some, MAMA MIA! and STARLIGHT EXPRESS might fall into this category. Myself, I'll admit to having seen SHENANDOAH many, many times because of my love for the source material and for John Cullum.