This topic seems to crop up all the time here, the AIC is quite clear in it's intent and it's been around for ages. The message should be clear to all us by now. Don't pitch up in a busy TMA with minimum fuel reserves, simple as that.
20-30 miniutes holding can come out of nowhere at LHR nowadays and personally I like to be able to do that and still have the alternate up my sleeve.
In fact due to the security situation at the moment we are being told to take an hours holding fuel to LHR to cope with the chaos should an incident close the airport.