The biggest advantage I found for speccing this way is that both specs value gear similiarly, and because of the points in suppression less itemization needs to be used for +hit. Both of these specs are hit capped at 11%, and value
SP > Haste > Crit Rating > Spirit > Int
With ratios of about
8 : 12 : 14 : 15 : 48 (in terms of their respective benefits)
So 8 spell power is worth 12 haste is worth 14 crit, etc.
This means you don't have to worry about gearing up two different specs, and can concentrate on maximizing your gear/gemming/enchants using the above weights.
The other advantage to this dual spec is that you have flexibility on boss fights. For fights where there is a lot of movement you can utilize your dots to help keep your dps up while running. For fights where there is little movement you can use the naturally higher sustained dps of the destro spec, and have a nice rain of fire for trash pulls.
Hope this helps someone.
 Fixed ratios to make more sense.
Last edited by Sarrael on Wed Aug 26, 2009 5:34 pm; edited 1 time in total