I personally run about 2000km a year and deadlift 145kg by the time of writing. Crossfit helps me to keep motivated, improve my personal bests and stay injury free. If you have not started yet these reasons might persuade you to include crossfit in your training routine.
I started crossfit in the middle of 2014 without taking classes and following the crossfit journal and getting a subscription to beyond the whiteboard. Since then, using the stronglifts 5x5 program, i was able to develop to a 100kg squat and 145kg Deadlift packing about ten kilograms on muscle from 80 to 90kg on my body. After running for one and a half years i got comments that I have gotten very skinny and looked pale. Not anymore. It depends on your eprsonal goals, but if you do not want to be in the 2:20 to 2:40 range of marathoners maybe consider your all around health in favour of the ultimate running physique.
The recovery times between my first marathon in Dublin in 2013 and my second in Frankfurt in 2014 were as different as day and light. In 2013 it took my about a month to recover fully from my ordeal, whereas in 2014 i was fine and out on the road again after a week. I hugely attribute this improvement to the time spent on the high intensity, short duration crossfit workouts.
I personally found to focus only on running can get very dull after a year, especially if you are not doing it in a running club. Crossfit gives you the opportunity to mix it up and have different levels on which you can challenge yourself, when you are going through a running slump and stay motivated.
Mixing it up also avoids to become a "running bore" as Ronnie O'Sullivan, snooker champion and passionate runner, describes a person in his book who only talks about running and nothing else anymore. Crossfit is accessible to a wider range of people and also hip and trendy at the moment.