If you are a professional walleye fisherman or deep sea fisherman, I can say yes, it's worth it. The duel freq. is great in deep water, salt and if you are going to be fishing amongst cover in the mid depth range for lets say eyes.
If you are very,(and I stress very) knowledgeable about your graph, know what you are looking at and then know how to use your graph to stay on top of what you see,,, it's worth it.
The difference in the frequencies, is for the person that uses them in different types of water environments. Lets say your using your graph in salt water over 200ft. A lower freq. will be better for you because the lower freq. will penetrate the denser water column then a high freq. The same thing is true if your using the graph in shallower water with a lot of vegetation. Low freq. will penetrate the vegetation better then high freq. You have,,, and I stress HAVE,,, to be knowledgeable about your unit.
I'm not a pro at this.
I will say though,,,,, I have hundreds and hundreds of hours watching that screen in front of me. The best teacher is experience. If you are fishing fresh water??? Don't worry about duef freq.
Good question.