I think the only time I've seen engineers add feature/code creep is when it's "filling the gaps" that others didn't think of. Many times ideas are presented to engineers in their unfinished format. It's often up to engineers and QA to find the areas where that idea needs to be fleshed out. Fleshing out an idea, I would argue, isn't feature creep but trying to make an added not suck.
Yup. When you've got a system that, as implemented, gets you 99% of the way to a nifty but unrequired feature? It's dreadfully tempting to go that last 1%. But somehow that 1% balloons into being something that you have to support and the UI turns out messier than you expected and yadda yadda yadda.
