I suspect monospaced fonts were a programmer's preference as a carry-over from the text-based DOS days.
On the other hand, I, myself, have tried Verdana and a couple other recommended proportional fonts, but I couldn't deal with the change. My eye is too well trained for monospace. Languages heavy on symbols, like: C/C++, C#, Perl, etc., look too different for me. The placement of symbols makes the code look completely different.
