The rule for subdual in OD&D says (Monsters & Treasure, p. 12/13):
Subduing Dragons: Any attack may be to subdue rather than to kill, but this intent must be announced before melee begins. When intent to subdue is announced, hits scored upon the Dragon are counted as subduing rather than killing points. Each round of melee the number of points scored in hits is ratioed over the total number the Dragon has (hit point total), the hits obtained being stated as a percentile of the total possible, i.e. 12%, 67%, etc. The percentile dice are then rolled to determine if the Dragon has been subdued. A roll equal to or less than the percentage of hits already obtained means the Dragon is subdued. For example:
A “Very Old” 11 Hit Dice Red Dragon is encountered asleep in its cavernous lair. Three fighters creep in and strike to subdue. All three hit, scoring respectively 2,3, and 6 points, or 11 points total. 11 ratioed over 66 (the number of hit points the Dragon can absorb before being killed or in this case subdued) is 1/6th or 17%. The referee checks to determine if the Dragon is subdued and rolls over 17 on the percentile dice. The Dragon is not subdued, and a check is then made to see whether he will bite or use his breath weapon during the second melee round. The result indicates he will breathe. The attackers strike again and once more all hit for a total of 12 points. The Dragon breathes and as none make their saving throws the attackers are all killed for they take 66 points of damage from Dragon fire. Subsequently, the referee rolls 01 on the percentile dice (any roll up to 34 would have indicated success) indicating that had the attackers survived, they would have subdued the Red Dragon that turn.
As I read this, subdual damage is cumulated just like normal damage would. The fighters deal 11 points of it in the first round, and 12 points of it in the second, together 23 points in the second, for a 23/66 or 34% (rounded down) chance of subduing it.
So, it will take as long to kill the dragon with normal damage, as it would take to assuredly subdue it (when he has received 66 subdual damage, your chance to subdue it is 100%). But on the way there, you have multiple opportunities to end the fight early by subduing the dragon, so the fight is a lot less dangerous.
On top of that, a subdued dragon is worth 500 to 1,000 gp per hit it can take, so in the example above at least 33,000 gp, and the character can opt to keep it in his service, and you get a free dragon out that easier fight. Dragons are great -- Mordenkainen had a pair of Red ones, and Robilar had a pair of Green ones, and they loved having them.
I would think it should be harder and more dangerous to subdue the dragon to get those benefits, not easier and less dangerous. Given these rules, why would anyone in their right mind ever opt to not subdue the dragon?