It's exactly like Ozz says. If you have spare cash because you're on a low interest rate at the moment don't assume the best option is to overpay straight into your mortgage.
Jurgen's situation is tantamount to the same thing - he's deliberately choosing not to pay off any more capital than he has to (in his case by having an interest only mortgage - of course there is a bit of a gamble with house prices if you go interest only, but that's the trade off).
If you can earn more interest on that extra money by saving it elsewhere than you would save by paying off extra on your mortgage, then save it elsewhere, and pay off in a bigger lump sum at the time the situation reverses.
Due to my silly, low interest rate, I'm actually banking away loads of money at the moment. It's going into long term savings where I'm earning ~5% interest (net 3% after tax, of course). If I was paying it into my mortgage, I'd only be saving the 1% interest that I'm paying on the mortgage.