One of the things that I immensely enjoy is having the group fight members of "good" races: two human nations go to war and the characters are stuck in the middle, a tribe of reclusive elves are killing travelers because they want to keep civilization from encroaching on the wilderness, dwarves are assaulting people from an organization they consider corrupt, halfling thieves are starting a turf war with a thieves' guild dominated by humans and half-orcs in a large city.
Part of the appeal to me is that there's a reversal of expectations, there's an unspoken expectation that virtually every members of a PC race is good in alignment. Another big part of it is that it shakes up player familiarity about exactly what they are fighting--everyone has fought an orc tribe lead by an orc barbarian and/or shaman. A surprising number of players haven't been in campaigns where they battle an elf tribe lead by an elven druid, a dwarf clan dominated by an order of rangers and inquisitors, or a band of merry halfling thieves lead by a troupe of bards.
NPCs with the warrior class make fine low-level "monsters" on par with kobolds, goblins, and orcs. Even those with one level in a PC class rank in at CR 1/2, so it's fine to have a battle with several warriors and someone with a class level even as a 1st-level challenge. There's also such a huge variety in class options that players won't necessarily know what they're up against even if they know that it's a halfling rogue or an elf druid.
Had a lot of fun with them this weekend.
