Since getting a wireless network card (and getting into wireless networks) I've used wpa_supplicant on Slackware and after the initial configuration using it has been pretty much painless. Configuring wpa_supplicant wasn't exactly difficult, but did take some time at first with a Broadcom card and ndiswrapper (before the native kernel module and fwcutter). However I think those who choose Slackware instead of, say, Fedora are already geared towards making some configuration changes themselves (directly into the configuration files) rather than always using point-and-click along with graphical-only tools. I don't carry my Slackware-loaded computer around, so I don't need to worry about wireless hotspots, but from what I've read wpa_supplicant should be capable of doing that sort of roaming too..so there you go, automation
On the other hand I use Ubuntu too, which means I'm using the NetworkManager (through nm-applet) with wireless networks. Works like a charm, and this new 0.7 version fixed an issue I had with older versions, so in short I like it very much -- eases up life. It needs to be configured (fill in passwords, perhaps keyring password, ...) at first just like wpa_supplicant, and after that you can pretty much forget it. But since it's so easy to use (partly because of that, at least) I have absolutely no idea how, or if it's even possible, to configure it if I don't happen to have X running, which is bad
. I can't rely on having X at hand always, and sometimes I don't even want to run it -- but still need to get connected. I assume it can be configured/used even without X, but have no idea how to do it; that means it would take time to study the thing, which is exactly the opposite of what it was supposed to be (a thing so easy that you don't need to learn to configure). On the other hand wpa_supplicant is fairly straightforward to configure and once it's done, can be used directly, trough scripts, in command line or by clicking a nice icon on a graphical desktop ("icon" => script, launcher, link, ...)
So even though I absolutely agree that it's good to have options and that things shouldn't be so darn hard for a beginner, I think wpa_supplicant fits into Slackware way better than NetworkManager (even before we start to talk about dependencies, installation etc.) If you cared to take the time and install Slackware, configure the system and still used it, how come you woulnd't take the time to configure wpa_supplicant (or just install one of the fancy graphical KDE apps for that matter, if you liked KDE)?