You're Better Than Plain Mashed Potatoes
What makes potatoes great? They're a vehicle for all of your other favorite flavors (cheese). That's their life's purpose. When you leave mashed potatoes plain or casually throw some butter on top, you are robbing them of that purpose (to hold cheese). If you are someone who has been guilty of this, don't worry. It's not so much that you're wrong, as that your potatoes are not living up to their potential.
Making homemade mashed potatoes is work. For enough mashed potatoes to feed a Thanksgiving dinner full of people you need (roughly) 700 potatoes. Your best bet in this scenario would be to buy one of those gigantic 5-lb. bags they sell at the grocery store and carry it all the way to your car and then all the way home. Once you dramatically swing the bag over your shoulder and plop it on the kitchen table, you have to peel them. Peeling potatoes is used as a form of punishment at boot camp in basically every movie.
To cook them, you have to take a kiddie pool-sized pot and fill it with the potatoes and enough water to cover them, throw in handfuls of salt, and turn up the heat until it boils—about one or two years. Once they are cooked, if you successfully drain the water from the pot without burning yourself, you're now ready to chop them up with a giant knife. I love knives because no matter how careful you are, there is always a chance you will slice your finger off and that is exactly the kind of anxiety I need to keep me alert in the kitchen. Once chopped, you're at the smushing stage (more manual labor). Grab your potato masher and use those biceps as you slam the weirdly shaped metal thing down into your bowl of potato chunks. If you like your mashed potatoes smooth, you will probably still want to smush them, but then transfer the globs to a food processor.
Once you have your finished mashed potatoes, you're tired. Even writing out every step out like this makes me tired. You also have a sink full of dishes you have to clean. You worked hard and want to take a nap. In a final act of desperation, you throw some salt on them. Maybe a little pepper. Maybe some butter. Ideally, some sort of milk or cream. Then you leave them in the bowl until meal time. This is where you are wrong. You broke a sweat for these potatoes. You trained to run this marathon all day and stopped before the finish line. Every bit of effort you put into this dish is about to be wasted when people use it as a vessel to hold a little puddle of sad-soupy gravy.
I would challenge you to think of it this way: If you went through the trouble of hauling, peeling, cooking, and smushing those potatoes, then you owe it to yourself to go the extra step to make sure your guests know exactly where to direct their compliments (to you). What works well in potatoes? Aforementioned cheese. Lots of different kinds of cheese. Think bigger than cheddar. Last Thanksgiving, I added ricotta, sour cream, chives, parsley, and garlic to my mashed potatoes and everyone lost their minds. If you wanted to go in a different direction, you could mix up some goat cheese, caramelized onions, and a little garlic. Goat cheese and caramelized onions are classically delicious flavors that your blessed potatoes get to carry now. If you're not a huge cheese fan, I get it! (I don't.) The good news is that there are plenty of spices and fresh herbs that will also do the trick. Throw some smoked paprika in there to add depth. Sprinkle some curry on it. If you're feeling fancy, shave some truffle on top! You could even toss in some chopped up bacon, and I would be proud that you went the extra mile. If flavor is the only thing standing in the way of making you and those around you happy, then don't be a Grinch this holiday season.
At the end of the day this is your life, your meal, your potatoes, your reputation. Far be it for me to tell anyone what to do while I sit on my bed eating dry cereal from the box. I don't want to judge. I'm not saying that you can't just leave your mashed potatoes plain. However, I am saying that if you knowingly choose to leave your mashed potatoes plain, then I truly don't understand your motivations.