You contended that a comma is clearly called before the conjunction “but” in the following sentence:
“We arrived exactly at noon as agreed upon yesterday but they arrived over an hour late.”
Following that contention, that sentence should be reconstructed as follows:
“We arrived exactly at noon as agreed upon yesterday, but they arrived over an hour late.”
This is indeed the formal, general rule for linking two independent clauses, but in my own experience, it is largely ignored when the two independent clauses are not only short and uncomplicated but also balanced in construction. This is precisely why I didn’t use the comma in this sentence that I used to clarify Rule 2 in my previous posting, as follows:
“We arrived exactly at noon as agreed upon yesterday but they arrived over an hour late.”
See how using the comma looks more and more uncalled for—and I must say overly fastidious—as the balanced, independent clauses get shorter:
“We arrived exactly at noon but they arrived over an hour late.”
Compare with a comma before “but”: “We arrived exactly at noon, but they arrived over an hour late.”
“We arrived at noon but they arrived late.”
Compare with a comma before “but”: “We arrived at noon, but they arrived late.”
“We arrived on time but they arrived late.”
Compare with a comma before “but”: “We arrived on time, but they arrived late.”
“We were on time but they were late.”
Compare with a comma before “but”: “We were on time, but they were late.”
It’s an entirely different matter, of course, when the independent clauses are long and complicated and when they are not balanced in their structure. This time, the comma before the “but” (and before the other coordinating conjunctions for that matter) becomes absolutely mandatory for clarity’s sake.
Try making sense of the following sentence with long, unbalanced independent clauses:
“We arrived at the airport on time after having to replace a nasty flat tire along the way for almost 20 minutes but our companions in another car that left 30 minutes earlier never made to the airport because they made a wrong exit at the expressway and got hopelessly lost.”
Now see how the sentence reads much more easily and clearly with a comma before the coordinating conjunction “but”:
“We arrived at the airport on time after having to replace a nasty flat tire along the way for almost 20 minutes, but our companions in another car that left 30 minutes earlier never made it to the airport because they made a wrong exit at the expressway and got hopelessly lost.”