So Sharjah taxis have implemented the Dhs20 surcharge for going to Dubai but, as far as I have seen, Dubai taxis have not implemented the charge the other way.
The charge was ostensibly to make it easier on drivers reluctant to brave the traffic and not be able to return with a fare due to the odd rule that cabs can't pick up fares in other emirates. A driver rewarded is a driver keen to serve, is the theory, no?
As Gulf News (690g) asserts, in the report linked above: "Taxi companies in both Sharjah and Dubai lobbied for the new flag-fall rate. The decision was taken to provide an incentive to taxi drivers, who have sometimes refused to make the trip between the two emirates, especially during peak hours or on holidays, leaving passengers helpless."
The truth, certainly according to the cabbies I've spoken to, is a little less poetic.
Out of the Dhs20, the cabbie gets only Dhs5. Dhs13 goes directly to the company and the rest gets eaten in 'fees and commissions'.
As usual, the cabbies get screwed over, the cab companies take our money. We get the same old woeful levels of service but get gouged again.