I cringe every time I see people who are appending two scalar parameters for every vector 2 control they need, especially UV stuff.
It’s not that big a deal, but when you have a dozen of them, it really starts to add up in your material instances.
At the same time, unpacking (ComponentMask) can sometimes end up being an operation on the GPU 
And the functions that send them (Set Vector Value) become more error-prone, as you have to store all other values somewhere in the Blueprint. Then you have to remember to send all 4 scalars when you update any of them.
It’s all about ease of use and readability vs. solving an actual performance issue. If there dozens of parameters, then sure. If a couple - I won’t put too much emphasis on such optimizations 
I only use component mask for my texture samples, so I can choose what channel to use for a given effect. I use the Append node for all my vector stuff. Keeping in mind, that main material is almost exclusively for use with particles in Cascade. I’ll make cleaner materials for use with Blueprint stuff.