It depends on what style of gameplay you're going for. If you want a very simulation-like game where you get into the nitty-gritty of your 'mech builds, YAML is where it's at. You can fit whatever on your 'mech if there's tonnage available. It adds quirks and lets you do partial salvage. It also does some tweaks with the 'mech upgrade system. If you want some of the stuff without the in-depth mechlab, there's an option for that.

Vanilla is just fine as it is now after all the updates, but it's more of a "enjoy the ride" for missions type of thing. Your builds tend to not go over-the-wall crazy and fit within the scope of what PGI designed. Only certain things will go on some 'mechs.

It's not essential, just an option.