Also from that RFC:

> Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.

I guess my point is that idempotence is the aspect that matters to clients - you can have side-effects if you really want, so long as the request can be safely repeated.