I have the fixed BD Zpoles and have never wished for any type of adjustment. I have had my set for going on 4+ years of hard use and they are a favorite of mine. I am 6'1" and the 130s are very comfortable for me. Also, being fixed, they are very strong.
I’ve not used fixed length trekking poles but mine have double stacked grips so when you go uphill you just move to the lower grip and don’t have to adjust. If the poles you’re looking at don’t have a second grip you could make one with hockey tape I suppose. With the double grips I find I never adjust my pole whereas with other poles in the past I was adjusting pretty often for ups and downs.
Adjustable is nice if you ever want to use them for a tarp pole. I always leave my poles set for going uphill and if I'm going downhill and need longer, I just palm the very top of the grip, might not be as optimal as adjusting but it's more convenient and gets the job done, I like the sound of that double gripped poles the one poster above mentioned though!
I agree with Realunlucky and Oldgoat…
A short hike up or down no adjustment needed, but when on a long descent or climb, its paramount for me to adjust the poles.
I also use the pole for a tarp for emergency shelter, so it needs to adjust for that.
Personal preference.