The -dev extension has the "phpdbg" debugger executable, so it can process PHP code without needing one of the other executables from -cgi, -cli, -mod, or -fpm. All of them can run without -ext so it not required per se, but they could all benefit from it. I would imagine that someone would target one of these four depending on their choice of web server and likely add -ext for database access and the other goodies on their production server, and use -dev plus -ext on their development box.
I did notice as I was looking through the file lists that /usr/local/etc/pear.conf is in -mod instead of -dev, so I'll need to fix the build script and re-upload it later today.
Are there stats on which if any are being downloaded? I'm curious to know if there is anyone besides me using TC for LAMP.