Is there any special benefit to defending a "tall" gym?

There are no direct benefits for having a tall gym: no defensive bonuses, change in CP decay, rewards etc.

There is a practical advantage in the sense that a gym being tall can offput people from attacking the gym. The higher CP means that the gym would likely be more difficult to defeat since the defenders are assumingly stronger both in terms of individual CP and motivation. Generally gyms that aren't tall either have fewer defenders, less motivation or both, making them easier targets. It's often said that the best way to defend a gym is to discourage opponents from attacking in the first place!