All things in moderation, including moderation. (Author unknown.)
I consider myself very lucky to have developed software using assembler (a bit-slice processor in the late 70's, the 6809, x86 and 68000 in the 80's and 90's), then C (late 70's to the late 90's) and finally Perl (late 90's until now). I don't need to build my own hardware or write my own assembler or compiler, but I know enough about doing those things that I appreciate what's going on underneath. So by all means build on the work that others have done before you, as long as you appreciate the history that has gone before you.
And I have made my own pastry and pizza dough, from scratch -- yes, it's a little more work, but it's worth it if you have the time. And when time is short, it's fine to use a good quality prepared ingredient. Just as long as you have a deep understanding of what the meal (or the code) is made from.
Alex / talexb / Toronto
"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds