I remember why I switched to Google: 1. speed, 2. simplicity. Relevance, meh. Never noticed much difference there. This led me to believe that PageRank was more about marketing (brilliant marketing) than technical edge.
Initially the no. 1 reason for me was peer pressure. It was simply unacceptable to use something other than Google in tech circles at some point. It was like having an @aol.com email address. If your users are that passionate, you're doing something right. After I gave it a go for a while, the other reasons you list made me stay.
From what I recall, it was also the most programmer-friendly search engine at the time.
http://www.bing.com/search?q=DGGEVX
http://duckduckgo.com/?q=DGGEVX
http://www.google.com/#q=DGGEVX
This is purely subjective, but Google is the only one that links to the Netlib 'real' LAPACK, straight to the file/documentation in question. The others have mixtures of Java packages and other examples...