The truth is, what makes me nervous about a high volume of plug-ins on a site is the sad reality that many plug-ins are simply poorly developed. Even if they work in a sandbox, they often don't work well at a high traffic scale. You might get away with a few so-so plug-ins, but the odds of a badly developed plug-in taxing your site get higher the more plug-ins you install, and the more bad plug-ins you run, the worse the problem becomes.

Even many well developed plug-ins have become insanely bloated, adding layers of code to WordPress execution for features most users will never touch. And I speak as the author of about 15 publicly available plug-ins.

The reason I built Simple Local Avatars, for instance, is because *popular* plug-ins that were once simple solutions (like "Add Local Avatar") have become - in my humble opinion - far too heavy and bloated. I don't want that weight sitting on top of WordPress. I wanted a plug-in that added an avatar upload field to the author's profile, and would use that avatar - if available - when an avatar was called. No new setting screens (there is a single new setting option in the Avatar section of Discussion -- where it belongs -- that lets users determine whether lower permission roles are allowed to upload avatars), no third party API communication with social media sites, no new dashboard widgets promoting the author, no new folders in wp-content. I had to build the answer.

I'm not sure what the answer is, but I often see adding lots of plug-ins to WordPress as akin - in many cases - to all the freeware software people used to install on Windows. We all know how that ended up working out.