You are right. There are very few occasions for most of us which would require a PC and where a black Argyle would not do. However, the PC has become the de facto standard for evening attire in many circles here in the US. Events like St. Andrew's Balls and Rabbie Burns Nights will see more PCs than black Argyles. Attend in an Argyle and you might be appropriate, but you also might be alone.
But then the PC has also become overwhelmingly popular for day weddings as well, where by traditional standards the Argyle or even tweed would be far more appropriate, depending on the level of formality. We tend to overdo it here. It's all about the bling. Fly plaids are another example, already discussed in this and many previous threads.