Is folding with a spatula really necessary? In most recipes involving whipped egg whites or whipped cream, you are instructed to fold a the whipped mixture in parts (thirds) into the non-whipped mixture using a spatula. For example, angel food cake or souffle.

But, it is easy to deflate the mixture too much when using a spatula, especially for someone inexperienced, so why not use a whisk? Other than scraping the sides/bottom of the bowl, I don't see any reason why you shouldn't, and you could also use a folding motion while also preventing the mixture from deflating, and combines them faster.  Personally, I prefer a whisk.

Is there any other reason to use a spatula over a whisk, or is it just conventional practice?

In other words, is convention correct (based off your experiences)?
The mechanism of a whisk is very different from a spatula. One is able to very gently fold the ingredients with a spatula while minimizing mixing. A whisk's tines will cause much more damage to the whipped mixture, deflating the fragile bubbles you worked so hard to create. It's really worth practicing with the correct tool. It won't take you long to master this. 

Simply cut through the ingredients with the spatula and lift about 1/3 of the mix from the bottom and gently fold over the top, then turn the bowl and repeat. Fold as little as possible to just combine the ingredients. In some recipes complete mixing is not even necessary.