I personally do believe there's something to be said for giving a kid a massively obsolete computer of their own to muck with if the kid shows some interest in seriously engaging with "computers", something they can experiment with/take apart/break/repair without dire consequences. However, I do say that with the asterisk that the machines that qualify as the best computer learning tools are the ones *least like Macintoshes*.
The most educational machines I ever owned were a pair of TRS-80 Model I's I bought at a garage sale. They were a bizarre combination of a surprisingly engaging user interface and DOS (much friendlier than MS-DOS or CP/M in many respects) and *really hackish* (or, as Apple would paint it, "minimalistic and elegant") hardware. In addition to the casual machine-language futzing I hit those machines with the soldering iron, pressed together cables with vice grips, interfaced them to hardware stolen from more modern machines, and all of these things were surprisingly easy because the Model I hardware was so simple and forgiving yet, believe it or not, "standards compliant". (Much hardware harvested from an IBM PC works in a TRS-80 with little or no work.) Those machines were weird enough to be compelling yet simple enough to *fully understand*. Undoubtedly everyone around my age has some 8-bit wonder that they could similarly wax poetically about. Those machines taught you to about *computers* simply from using them. You had it shoved down your throat, and if you had the temper for it you enjoyed it.
The problem with an old Macintosh is it's *not* going to do the same thing unless you really force it. It's designed to be an appliance that runs applications. Using it will teach you what it takes to use the particular applications you run, IE, the syntax trivia and mouse motor memory it takes to type a letter or click a Stickybear. It won't (as a consequence of its arcane primitiveness) accidentally teach you how a computer *works*, and an "application oriented" person who doesn't want to learn a computer is going to be put off when they discover that they can't run the applications that the "application-using world" expects them to be able to run. ("What, you can't open that .doc file I sent you?" "Why aren't you on Facebook... your computer can't load it? Lame.") And to top it off, what hard-computer skills someone *does* learn from mucking with an old Mac won't translate well to anything else. ("Uhm, rebuild your desktop? You don't have to do that?" "Where's the popup to allocate memory to Netscape in OS X? There isn't one?" "Wait, your computer *doesn't* crash half the time when you switch from your web browser to your email program while a page is loading? I gave up even trying to do that on mine...") There just isn't the push to take it to "the next level" with a Macintosh... indeed, outside of things like Hypercard the learning curve for programming the Classic MacOS is one of the steepest ones there is. As for hardware hacking Macintosh-compatible (68k-early PPC-vintage) hardware is rare and getting rarer. Heck, nearly every plug on a beige Macintosh besides Ethernet is *completely* absent from modern systems. It's still fairly trivial to maintain a 15 year old PC even if you're limited to going to "Fry's" or another Big Box retailer (Although the "why bother?" question comes up) but a similar age Mac? Not so much. It gets better of course once you break the "iMac threshold", but if you're above that line you should be running OS X if you possibly can, thus making for a different discussion. Then it's no longer an "Ancient Mac", it's just an annoyingly slow and crufty "modern" one.
It just really doesn't seem worth sticking the "needy" with a 68k/early PPC when, as stated, a decent caliber Pentium III or 4 is just as "cheap"... arguably cheaper since a machine like that will easily run a modern Linux distribution with all-free apps while the Mac will almost certainly require at least some software that is either purchased or... scrounged (read "pirated"?). The Pentium III+ will give someone a decent Internet experience *and* at the same time be more accessible for hardware hackery... despite being newer it's actually more "disposable". Seems like a no-brainer to me. If you really want them to run Stickybear slap BasiliskII or SheepShaver on it, it'll run it faster than the real thing.
Anyway... Bleah. :^b