Assuming a high quality woofer and good crossover design, a 1000 Hz crossover is appropriate for a 15" woofer and 90 degree horn. Altec used a 1200 Hz crossover for the 15" woofer and horn in their epic Model 19, which was said to be a significant improvement over its predecessor the Valencia (800 Hz crossover). The rationale behind the higher crossover point is as Shadorne said: Matching up the radiation patterns. In my opinion it's worth doing, and the configurations Tyler is using in their Pro Dynamics line are well thought-out.
Using a 15 inch woofer to do mid range
I have noticed that Tyler Acoustics are using a 15 inch to do midrange down on their Pro- Dynamics speakers PD30 & PD 15. Isn't this pushing the 15 inch to far?