Aftermarket parts are optional parts, period. I don't use many, to be honest, and am a firm believer that upping your finishing skills more than makes up for any perceived lack of "extra" parts. Honestly, today's models are SO detailed, SO good, that aftermarket items are very often just "vanity purchases" which convince the buyer that their model will be somehow just alittle different than the rest of the same builds out there. I do that too.
But seriously, learning how to finish a model with more realism can acheive wonders for the "curb appeal".