Of course they are not necessary. It really narrows down to how you're going to arrange your food.
If you refuse to eat fruits, vegetables, grains, nuts and beans, then you would probably need something to compensate for the missing elements.
When consuming variety of whole plant based foods, both raw and cooked, you don't need to have any replacements.
These fake meats/substitutes aren't expensive anymore. Perhaps there are some brands that are famous or are still considered high quality in sense of taste and nutrition, but most of them are still cheap. Especially if you take into consideration that there aren't any subsidies for these, unlike meat/milk products.
Also, you can always make your own 'fake meats' out of basic ingredients. For example, to make seitan, all you need is flour. It is the same for the rest, where you just need to learn the skill and have few trials & errors. With the right combination and spices to your taste, you can have something that looks, feels and tastes similar to meat for a very cheap price.
Remember that the Veganism means stopping all unnecessary harm to the animals. So if these fake meats are going to be a choice for some people over meat, it is already good enough. Ecology and nutrition comes afterwards, and all these brands/products are a bit different. Some are considered real 'junk food' and they aren't hiding it and other are carefully sourced (often clean, organic and fair trade) taking into consideration the health benefits of each ingredient.