I've been to Daytona and Myrtle Beach & Laconia in Stop & Go traffic with Daytona being the WORST (for heat & humidity) and even with My 155 C.I. S&S Motor the heat hasn't been too much of a problem-or should I say "an issue" so with that said......I cant say that the BDMs *need* an oil cooler unless that same type of heat ,traffic and humidity are a Constant weather condition that you ride in All The Time .
Yes they work well , and are neat looking units-and "cool " oil is your engines friend .
But the 103,107,111,117 & 124 C.I. S&S engines are pretty robust to begin with (actually they are The Finest) and warm oil is better than COLD oil for performance issues.
As far as the internal build design of the BDM oil cooler-it's a partition divided "coil "....the warm oil coming INTO the inlet line-pumped Up and over the partition and into the coiled return line that runs down the length of the cooler and is attached to the internal wall of the face of the oil cooler that gets the air flowing over the ribbed outer cooler body-thus reducing the temperature of the oil that is flowing thru the coiled internal line attached to the cooler wall.

Hope this helps you out as far as build design of that unit.
I saw this information I've shared with you dudes 1st hand in an oil cooler that was damaged & taken off(and cracked open) on a BDM Ridgeback that had been in an accident(the guy rear ended a car in front of him while doing a holeshot

)
Vince