nice idea in principle, shame about the impracticability

Specifically the idea is to protect the user. All users of a GPL'd work can get the source code and fix/ improve it (for non-programmers and large organisations that will usually mean "pay someone to...")

With the non-copyleft Free licenses only the people you distribute to get this right, everybody else can be stripped of that right out of malice, in pursuit of profit, or most likely of all, simple laziness.

Of course to be effective copylefts must be enforced, for example Firefox is licensed under the MPL. You can get the entire source code to the Firefox on your Windows, Mac or whatever system. But the "Timberwolf" third party port of Firefox code to the Amiga OS 4.0 base simply doesn't provide any source code at all. Its authors have simply claimed that years of "alpha" and "beta" releases don't qualify as needing source code, and so far nobody has sent any lawyers after them, so their users don't get the freedom that they're owed in principle.