There are a couple of issues here.
Cabbies
The first thing you need to know, Morbius, is that the original removal of plural words from the list was done by an automatic process - a program deleted every word that was the same as another word with S appended to it. This produced a number of obvious errors - news is not the plural of new, needless is not the plural of needles, etc. So I tried to find such cases and restore the words, but some were inevitably overlooked. The correction of such anomalies relies on people raising issues, such as this one, and me considering the word and deciding how to handle it. This has been going on throughout the life of Chihuahua. Cabbies is a case that has never been raised before as far as I can recall.
Words like this are particularly troublesome because of the two singular forms, cabby and cabbie. The approach I take with such words is to allow the plural if it seems that the -y version is used a lot more often than the -ie version, for the singular. For example, I allowed caddies (after some hesitation), because it seemed that caddy was favoured over caddie by about 50 to 1. On the other hand, there are situations, like pixie, yuppie and cookie, where the variant ending in Y is very rarely used, so it would surely be confusing to players if the plurals were allowed.
In this case, it appears that cabbie is used more often. The Corpus of Contemporary American English has 144 examples of cabby, but 341 of cabbie. The British National Corpus also has cabbie dominant, by 50 to 31. I think if I were to allow cabbies, it would create more discontent than it would remove. A player who had successfully played cabbie would very likely assume there was no point in trying cabbies, and I think the disappointment of someone who misses a common word, because they assumed it would be disallowed, would outweight the disappointment of someone who lowers their hit rate because they assume a word will be allowed.
Guts
This word was initially excluded, but was added in 2008. I felt sure I had posted some discussion of the word, but I can't find it. It was among a number of words ending in S that I was admitting, after a lot of discussion on the forum about the issue of plurals.
My thinking is that in the most frequent uses of the word guts, it is not as the plural of gut. Usually guts is either an informal term for the stomach, or a colloquialism for courage. In the latter case, it's certainly not the plural of gut. But in talking about the stomach, gut and guts tend to be used with the same meaning. A "pain in the gut" is the same as a "pain in the guts". We don't use the latter expression when the pain is more widespread. We don't say, "My pain in the guts is going away; it's just in one gut now."
Of course, gut can also be used in a more formal way for the intestine, and here guts could be used as its plural. A scientist who has been busy dissecting frogs may have a row of guts laid out on the laboratory bench. But this is a much more specialised usage.
So, I feel the acceptance of guts was justified, but I'm not sure about its common classification. Of course it is a common word, but sometimes when it is not so obvious why a word is acceptable, I have leaned towards making it rare. The other word that was allowed, as a common word, at the same time guts was admitted, was goods. This is probably less contentious, because good is normally not a noun at all, but an adjective. Even so, I'm sure there are players who miss out on playing goods because it looks like a plural.
...It's all too hard, really.