The first is that Verdana is really nicely hinted, which means that, unlike most other fonts out there, Verdana has the advantage of always technically looking sharp.
The second is that Verdana is mostly monolinear, like Courier or Andale or any of traditional monospaces, there's only the slightest bit of stroke variation (most of it optical), so you get style but you don't get the distraction (onscreen) of stroke modulation
The third is that most proportional fonts have a comparatively larger pitch, so you get more characters in each line, which is also what you start seeing in 'next gen monospaces' that seem to make better use of their allotted horizontal space (and seem less wonky in general).
The last, and arguably most important, is that almost all proportional fonts (verdana included) have got style. It's hard to emphasize enough that most of the arguments for Consolas or Inconsolata or Liberation or ____ are largely due to the fact that those fonts have an overt aesthetic agenda, which is sometimes not apparent in first-gen monospaces (although, to be fair, they and their brethren are stylized, it's just that we've grow numb to it)
I'm personally a monospace sorta person (i like fedra mono)--there are some really nice (alas, non-free) monos out there to choose between that are more fun to use than the more stoic traditional monospaces. Part of why proportional fonts are appealing is that we rarely need monospace fonts anymore: their use is largely vestigial (and aesthetic-driven) and also that many programming languages (python et al) often have highly literate code that benefits from typefaces designed with legibility in mind.
So i'm just sayin, i respect where you're coming from, but don't dismiss monos out of hand. There are nice ones which are coincidentally monospace, but aren't as frumpy or harsh as the monospace fonts that are typically advertised as 'best programming fonts.'