In crossword constructing, a POC (Plural Of Convenience) can be, well, convenient. I can take a 5-letter word, say, and instantly make it into a 6- or 7-letter one by just adding an S or an ES. How easy is that? I can vouch that it is temptingly so. As is often the case with temptations, however, there is a price to be paid for giving in.
The price for succumbing to the POC's siren song is a reduction in the puzzle's overall quality. After all, the constructor just used a short cut to make it easier to complete the grid. There was an increase in filled grid space without a commensurate increase in value.
The degree to which a puzzle's quality is impacted depends on what kind of POCs are used and how many times they appear in the puzzle (too many and it can become POC marked!).
A POC can be categorized into one of four levels, going from the hardly noticeable (Level 1) to a serious threat to the puzzle's overall integrity (Level 4).
Level 1 POC is the common-as-dirt kind, found in almost all puzzles. POCs at this level involve only a single, relatively short entry being pluralized to up its letter count One might see a Level 1 POC in a corner like this (the NE corner from my 11/8/12 LAT):
J E E R
A X L E
W A L L
F L I M S Y
There 'tis, ELLS. Hardly noticeable, but a POC nonetheless.
Level 2 gives the constructor two POCs in one. Two entries share an S at the end. This is more of a threat to the puzzle's quality because that "S" is the equivalent of what's called a "cheater square", or, more charitably, a "helper square". This is a black square that doesn't change the word count, it just makes it easier to fill in the grid. In other words, that shared final S in a Level 2 POC could be changed to a black square and then just clue the words as singular rather than plural. Have I been guilty of this more serious POC? Yup, sad to say. From the same puzzle, SE corner:
A Y E
O I L E R
U V U L A
T E M PS
That's just too easy and it's why I think a Level 2 POC is a more serious threat to a puzzle's quality. I was able to rework that corner to eliminate that two-for-one POC in about six or seven minutes. Wish I had done that before submitting it for publication.
Level 3 is when a long, non-theme entry is a POC. Say, for example, HEX BOLT fits beautifully with some fantastic crossing fill, but it's a letter short for that slot. No problem, just add an S. But now a much more integral part of the puzzle has been compromised than happens in Levels 1 & 2.
Level 4 POCs so compromise a puzzle's quality that I think that they should be deal killers. This is where a theme entry is a POC. The theme is the heart and soul of the puzzle and shouldn't have to rely on an S or ES tacked on to one of the theme entries to make the letter counts match. It's kinda like adding a gratuitous syllable in one of the lines of a Haiku poem to get the 5-7-5 syllable counts.
Have I been guilty of using a Level 3 or 4 POC in my puzzles? Nope, glad to say.
The other factor that can impact puzzle quality is how many POCs appear in the puzzle. Obviously, the more POCs, the greater the impact. Since the relative frequency of the letter S in standard English text is around 6%, I think any frequency significantly above this in a grid, say 12% or higher, shouts "POC Assisted" or even "POC Marked". (Only 4% of the 100 tiles in a Scrabble set are S's.)
Combining POC frequency with POC Levels offers a way to give each puzzle a POC Score. The POC Score could then be used along with more commonly discussed factors, like theme consistency, excess abbreviations, long partials, etc., to rate the puzzle's overall quality.