The main exception I would grant to this is when you have ingredients in a recipe that you need to figure out vegan substitutions for. In this case you do want something marketed as the vegan version of it.

Eggs, in particular, need a valid vegan substitute. As an aside, if we could veganize all the different incantations of an egg it would be ground-breaking. Alas, I fear we are doomed to have 5 different vegan products to replace those incantations of an egg. 1 for scrambled eggs, 1 for baking, 1 for emulsifying, 1 for adhesion, etc.