Blind pew,
What if someone used Heathrow as an alternate on your day. Would he be classed as foolish? As spamcandriver puts it, what if someone would block the runway at your alternate, just when you are diverting to it.
You never have a definite landing assured. But if you have been given an ETA and weather conditions are good than I don't see any objection of burning your alternate fuel at your destination.