I really like plane layers compared to poly pours on normal layers.. YES they need a bit of attention, and altium could easily just make them work 100% with a bit of work around the via clearance and plane connect stuff (which I have long given up on them ever doing) but I think the feature is definitely usable for real designs, if you keep in mind there's always a couple of things to manually check, and I don't see it going anywhere.
I have tried an experiment recently by doing a few of my recent 4 layer (outer tracks, inner planes) designs with poly pours on internal layers instead of plane layers. due to recommendations on this forum. Basically, I found it fiddly and annoying, and I needed to spend more time routing because of it.
For me, the whole magic of plane layers is they are inverted - so they can visually exist in the design and be properly visible without blocking everything else out wherever they go. This means if you have a split plane with a few different power domains on it, you can see EXACTLY where the edges of the plane is for the power domain you need, both when placing and moving a component, and when routing component pins down to the plane.
With poly pours on a normal layer, the plane becomes very hard to see through, and so having it at all visible means you can't see other things you need to see, like potential blocking objects on the other side of the board when you're trying to route something.. the alternative seems to be polygons in draft mode (which for me is not visible enough, may as well not be there) or just shelving all polygons (which means the polygons aren't even there to electrically interact with and you can't see them when you need to route to one...) or turning off the view of the layers you have planes on... again, working blind... I literally have no idea how you'd effectively do an 8 layer or more board with poly pours on normal layers. it would be a nightmare. you wouldn't be able to see anything and you wouldn't be able to target the net regions you needed to. And an added bonus is - if you want to work with online DRC, as soon as you drop a via while routing, with your power and ground planes enabled, BOOM, the whole screen turns bright green and you can't see anything!! the only alternative here seems to be turning the online DRC off.
Also, for plane layers, adjusting split plane relative placement becomes a matter of moving a line. with poly pour on normal layers it's all grabbing polygon edges and corners and trying to mash things into shape then extend other things to match while visually maintaining clearance. so fiddly!
In short, I think the only way I could effectively do a 12 layer board that needed for some reason to have poly pours instead of plane layers would be to do it with plane layers, finish it to the point I could release it that way, THEN add in normal layers with poly pours to match each of the original plane layers... then delete the plane layers.