Now this is going back many years for the 737-200 but I recall the basis of the 30 minutes fixed reserve (Australian CAA terminology for 30 minutes holding where it is expected the touch down will be with 30 minutes of fuel remaing) - was to cover a go-around at the flare plus a 1500 ft circuit on two engines to a five mile final (from outer marker) and with reverse thrust on landing - and the expectation the aircraft would have insufficient fuel remaining to taxi off the runway.
So although 30 minutes holding fuel (nominally 1200 kgs I think) sounds fine, in reality, with the enormous fuel flow rate under the conditions described above, you could be in real strife and quickly using up your 1200 kgs. And don't forget to allow for fuel gauge tolerances worst case in both tanks of three percent of the full tanks figure - around 135 kgs per wing tank - so you could be as low as 1200 kgs minus 270 kgs (if your luck is down) equals 930 kgs total fuel on board on the go-around...scary stuff