In the early 80s, my parents went to garage sales and bought me tons of used Lego sets. None of the sets were anywhere near complete, and most of them didn't even come with a picture of the set they belonged to. My parents just dumped everything into a large bin and gave me no instructions other than what the shape of the blocks themselves suggested. There were parts of cars, parts of airplanes, parts of castles and…
I think it actually takes more creativity to re-purpose the 'heavily customized' sets. Here's some of those ship pieces you mentioned: http://farm6.staticflickr.com/5286/5377659338_18f315850a_z.j... As a counter point, I loved all of those specialized sets as a kid -- Blacktron and M-Tron were especially awesome. Plain rectangular bricks can be boring and need some custom pieces to spice things up.
If you got bored with plain blocks and had to "spice things up" with custom pieces, it means you had plenty of plain blocks at your disposal. That's how I like my Lego sets, too. A large number of plain blocks with a few custom blocks here and there. Not custom blocks galore like some of the newer sets tend to be.