I've only posted two WIP threads here on the forums, mostly just posted finished stuff.
One WIP covered an involved 1/700th kit bash of the WW2 carrier Enterprise in the Ships forum. I showed how I modifed the kit pieces with stock plastic to get the model to the point in the carrier's life I wanted to represent. It didn't generate too many comments until I'd completed the project, so I don't know that the WIP was of much benefit to any forum readers. I suppose that the subject matter wasn't something many modelers had tried, or even cared to try, but I did have a lot of views as I posted. The finished model was well received.
My other WIP was in the Space forum covering a 1/48 Lunar Module project using various kinds of foils. It generated more comments, I suppose, since more space models require crinkled foils as part of the build. (Certainly, a LM does.)
That's my personal experience with WIP posts.
As a reader, there've been a couple WIPs that have kept my interest, but mostly I've just wanted to see completed builds. The completed models can be good inspirational/instructional lessons in their own right. But unless there's a particular kit, or specific subject/technique I might need help with, many WIPs tend to take up lots of time scrolling through the pages and I don't always have the time, on a given forum visit, to go through it all.
But then, that's just me.
If I have a concern about either type of posts, it would have to be the poor photography on many of them. That's a little unfair since I've made my living with photography for over 30 years, and not every model builder is a pro photographer. Even with that, if the shots aren't very good, thay aren't worth much to the forum reader. I've posted and contributed to several threads on photo tips, and many photos posted are very good. However...
Builders all want to participate in the forums and should always be encouraged, blurry and poor exposures not withstanding.
Everbody should have fun.