Just to clear things up a bit, the adventure in question doesn't have a bunch of mid paragon level guards just standing there on duty or anything like that.
It does have an assassination attempt and a bunch of pirates at the level, which I have no problem. I'd assume that these are no ordinary pirates, but one of the "finest" pirate crews out there, a true terror of the sea.
It should also be noted that in 4th edition equal level npcs are by no means true equals to the PCs. PCs are more like elites of their level. And a 15th level minion certainly isn't really a 15th level character. He's more like a tenth or eighth level character that is stated differently because at that level he represents a different kind of challange to the PC's.
Also I'd say that 15th leve is, over all, of a different quality than it was in other editions. In older editions 15th level was the beginning of the very high level, while today a 15th level char is only at exactly mid level.
The bottom line is that for me normal human and humanoid npcs of paragon level and minions into early epic level are ok if they represent elites and exceptional groups.
I'd be willing to even accept groups of epic level npcs if they represent ruly legendary organisations. An example for this would be a truly exalted order of knights, whose normal members could be about one hundret epic level minions in relation to epic PC's (paragon characters in "real" levels) and whose inner circle would be about 10 to 20 epic npcs, maybe led by an elite.