1. The origin of orcs both in LOTR and DND set the bar for me. In the former, the word 'orc' is sometimes used interchangeably with 'goblin', and they are simply bogeymen: creatures of night and darkness who live underground, eat people, and serve as the armies of the world's dark lords alongside other 'bogeymen' monsters like trolls, ogres, etc. Originally, DND just picked up that trope and ran with it, and that's fine- they were 'fodder' enemies, like the stormtroopers from SW, and unredeemably evil. It was 'in their nature,' sort of along the lines of the parable of the scorpion and the frog, and I don't see a problem with that.
2. The only reasons I see people wanting to play 'evil' humanoid types or monsters as good is a) because they have some social statement to make, b) they are bored with the 'regular' choices. I don't even really like Tieflings and Dragonborn, but I'll admit that my love of cats has made me a little partial to Tabaxi, although that doesn't mean I would necessarily allow them as a player choice in a Tolkienesque setting. Orcs, goblins, other humanoids are definitely OUT in my campaigns as PC choices, because I'm not going to try and warp the entire milieu of something like, say, Greyhawk, in order to incorporate them as anything other than implacable enemies of the free peoples of good alignment. Orcs walking into a free city populated by humans and demi-humans are going to be slain on sight by someone, and I don't feel obligated as a DM to try and invent clever workarounds for the benefit of one contrary player. Now if EVERYONE in the group wants to play an orc, that's something that can be worked with, but it's going to be a different campaign and setting altogether. Although I probably won't be running it, because I don't run 'PC's are evil' themed games.
3. In my games, players who try to make deals with humanoid tribes in dungeons usually come to regret their choice. When the alignment in the MM reads 'evil', that means evil. Sure, an intelligent creature can be reasoned with for its own benefit, but that doesn't mean there won't be a flat double cross at the earliest opportunity. The very definition of evil includes a lack of trustworthiness and integrity. Those orcs might make a deal with the players now and let them pass, but tomorrow, when the players are beat down from whipping the gnolls and carrying all that loot on their backs, those orcs are going to be back, hungry for shiny coin and man-flesh. You reap the benefit of your choices.
4. Since humans run the gamut of alignments, motivations, and such, I see little reason to repurpose monsters as player choices. If the player wants to be the noble savage, there are ways of accommodating that without letting her play a two-headed troll who's emo because the one-headed trolls picked on her as a kid. So on and so forth. Of course, different strokes for different folks. You do you in your games.