In History, it's not the adjuncting that is an issue in itself. Indeed, if the CV just lists classes taught, we may not even notice if the employment is part-time. It's the fact that if one is not in a TT position (or impressive FT professional job in the field) within a few years after completion of the degree, the application tends to be passed over in favor of more recent graduates. Three years used to be an informal benchmark, but this may have shifted a bit. Fundamentally, though, extensive/prolonged adjunct experience is often (albeit unjustly) seen as a negative rather than a positive.
A person as described above seems to lack ambition and/or creativity; if someone is just in a holding pattern, (especially if that is by the person's own admission unsustainable), then the longer they have been that way the more they look like they're just waiting for a miracle. A pro-active person would move on one way or another.
Isn't it sort of a waste of time trying to defend this? I'm not naive about this happening, I just think its absurd people would try to actually justify this attitude. Obviously in many cases someone adjuncting for a few years might not be a good candidate. If you are looking at a CV with no new work for five years, it is probably reasonable to assume someone is not an active scholar. Perhaps this is true of a lot of candidates who are working as adjuncts, in which case you can not consider them. What you're talking about here is just substituting a bunch of broad ideas you have about adjuncts in general for a consideration of qualifications and suitability for the job.
Taben mentioned the option of someone adjuncting while raising children. The point is, I think many people would rather see someone who has been actively getting on with life rather than someone who appears to be in limbo pending a full-time job in academia. Along the same line, the idea that someone is actively pursuing other options along with this one would look better than someone who is just holding their breath on this.
(Having never served on a search committee, for this, I'm just speculating. But I could see this thought process happening.)