This is the best comparison. Struts was great when it came out and inspired a lot of 2nd gen frameworks like Webwork. Webwork was very clever and much improved over struts. However, struts was far more popular. Bringing the two together created a better product that was adopted by the mainstream java web developers. It was a great way to get the developers that were stuck on struts (for whatever reason, most likely to support existing code bases) into a better design.

In about a year I could write the paragraph above substituting rails for struts, merb for webwork, and java for ruby.