Rel. Humidity vs dew point

[nogreenthumbs]

Why does everyone use the relative humidity values when talking about orchid culture rather than dew point?

The reason I ask is that 70% humidity at 60° is a very, VERY different humidity than 70% humidity at 80°. But if the dew point is 65° at both 65° and 85°, then the same amount of moisture is in the air. i would think that we are most concerned with moisture content.