Bouillon cubes are a quick cheap shortcut. Sure, if you want the most awesome possible taste in your soup, you'll make your own broth. And if you want something that's still awfully awesome, you'll use one of the better aseptic packaged or canned broths or stocks. (Yes, there's a difference between broth and stock).
However, when you find yourself a cup short or just generally running low on the good stuff, a cube is awfully handy.
Now, if you are starting from scratch at home to make stock or broth, it's much simpler to make it with meat. There's such a wide flavor profile that the meat adds that you can get away with fewer aromatics and vegetables. I've had great success with just using all of the inedibles left around after I'm done cooking. Bones, tendons, carrot peels/tips/leaves, a few bits of onion, etc. Whereas, to make good veg stock at home, you really need to use a bunch of different vegetables and be fairly careful with how you do it.
On the other hand, in a commercial facility, there's no excuse for not being able to turn out a decent veg broth. Especially considering that they can juice it up with "natural flavorings" that Food Inc. spends a lot of time explaining are really just last year's "artificial flavorings".
This is a source of annoyance for my wife. Because she is not a complete vegetarian, but has a number of religious observances that have her avoiding meat for periods of time. So, while chicken soup should probably use chicken stock, she gets ticked off when the vegetable soup uses chicken stock.
I'm not too fond of vegetable soup that ends up tasting like a chicken soup with the meat removed. Those kinda suck. The original sources for what became my potato-leek soup and my spaghetti-squash soup recipes both called for a small amount of chicken broth, but after I replaced the chicken broth with vegetable broth, it still tastes great. I don't really notice much of a difference, actually.
Recently, I ran out of the cheap vegetable bouillon cubes. It's not a huge problem, because I always keep an assortment of broths and bouillons and concentrates around. But I figured I should get some more. I went to both big "normal" grocery chains in the area -- Lucky's and Safeway.
It turns out that, either temporarily, or permanently, somebody has decided to stop carrying veg bouillon. Ugh. Because I can't find it. They've got the chicken and beef versions of the same brand. sigh