This sould clear it all up:
"There would be a possibility of conflict if traffic cruising above the transition altitude were on Regional QNH and aircraft only slightly above it were on the 1013 reference datum. For this reason there is a layer above the transition altitude at which cruising flight should not occur, to ensure satisfactory seperation of at least 500ft."
^^ an extract from the air pilots manual.
Because the transition altitude is at FL30(3000ft), you shouldnt cruise there/in that layer, because aircraft may be on QNH or 1013, making vertical seperation inaccurate.
So im guessing this is the reason that they dont reccomend FL30, but infact FL50... im sure this is right...?