I like slow cooked roast for tenderness but hate the taste of 'pot roast' vegetables. Is it really necessary to cook vegetables with the roast?
If you're doing it as a pot roast, the vegetables are mainly there to deliver flavour (unlike in a stew, where they're a major part of the dish). Sometimes they're eaten, sometimes discarded (which seems like a waste to me, so I would choose vegetables I'd want to eat). They take on a role similar to stock or a flavoured rub or oil when roasting in an oven.
On that basis you can generally omit them. One thing to watch out for though is that the vegetables can be used to lift the joint slightly, so it doesn't sit in a pool of fat*. In that case you'd want to replace them with something inert, or use them but discard them.
If you then find that the meat wants a little more flavour, you can always add a little of a suitable herb and some black peppercorns, to be discarded.
* Note that the term "pot roast" is used broadly, sometimes adding significant amounts of liquid and sometimes not. I don't know if this is a regional thing, but my answer is meant to cover both types.
To directly answer the question as it was originally posted before edit:
why does everyone add vegetables to slow cooker pot roast
To save time
It is not necessary to cook vegetables with meat. The vegetables do add flavor (a flavor that many appreciate) but I don't feel that is the main reason. Many people add potatoes to the pot roast while cooking. Potatoes don't add any appreciable flavor.
Instead, in my case I do it to save time. Sometimes I will plan my lunch meal on Sunday and put things in the pot to cook. By having everything in one pot, when I return from morning services I can just pull everything out of the pot and put it on the table. Based on services ending just about noon and the travel time home, this removes the need to wait for food to be prepared for the lunch meal.
It's also easier to do. Rather than put only the meat in one pot and then create more dirty dishes by cooking the vegetables separately, I can put them all in one pot and make cleanup time shorter.
So, to answer what I feel is a very opinion based question, "to save time".
I agree about those vegetables. Julia Child has a great solution.
Julia Child only adds aromatics like onions, celery, carrots, and garlic (never potatoes) to a roast in her pot roast recipes (see Beef Burgundy). Once the roast is finished, she strains-out the aromatics because they are now tasteless and mushy. She then sautes pearl onions in butter and adds those in just before serving. If you want potatoes, she cooks those separately or adds them to the pot roast about 2 hours before the roast is finished cooking.
Google "julia child beef bourguignon" for her recipe and many variations. BTW, her recipe is delicious, it is worth any extra time or expense to try at least once in your life.
Sometimes it's to lift the meat up so it's not cooking directly on the bottom of the pot. In any case, if you wrap your meat in parchment paper you can roast it in the oven and get pretty much the same effect as a pot roast.
You still need to add aromatics like onions to your roast if it's in the recipe though. That's a flavoring issue.