I'm a sort of accidental vegetarian, in that I never really decided to become one; the rest of my family is vegetarian and I stopped eating meat because it was inconvenient, and then gradually I lost interest in it.
I realized at some point MSG is really useful for adding umami flavor to things without adding other flavors, like seaweed or tomato or mushroom. It is wonderful for certain things, and makes some vegetarian dishes basically indistinguishable from those with meat. I think it's a little odd that people will go out of their way to add unusual strong-flavored ingredients for the umami, when you could just add MSG.
At the same time, MSG on its own to me has a really cheap umami quality. It reminds me of really cheap frozen dinners and bad food from elementary school. For awhile I couldn't get over that, and then eventually realized that when it's used in complement with the right things in the right amounts, it works great--you just need to figure out the right settings.
So sometimes those natural sources of umami just seem contorted and more trouble than they're worth. Other times, they seem to supply umami in a way that doesn't seem cheap or one-dimensional.
I suspect that there's some other component of many "umami" flavors we haven't discovered yet, or other tastes that technically aren't umami but would be identified as such currently. It's difficult for me to believe that MSG is really capturing most of what I like about savory dishes; I feel like something else is missing. I'm waiting for other amino acid salt receptors to be identified; I wouldn't be surprised if they're somehow linked in their activity to glutamate receptors or something of that sort.