There's things that the Deluxe/Voyager/Leader size classes do really well. One of those is the potential for remolding far exceeding what is feasible at the Core/Legend/Legion scale; not saying it's impossible (Core Ironhide and Ratchet proved that, though I'm sure that was planned into the mold from the outset), just that the practicality for all the variations and deep cuts we've gotten recently simply isn't there. (Straxus, Xaaron & Flame, Minerva/Elita-1/Strongarm, Skids/Medix/Crankcase, etc)
Another is poseability, which is great for static displays or photocomics.
But if we're just talking playability, smaller, less complicated scales are the way to go. Being able to transform them quickly is a plus (raise your hand if your Transformers had to have a force field during transformation to keep your siblings or friends from taking them out while between modes), and being able to set up a battle in a relatively small space is a necessity for most of us. And that's not even getting into things like playsets and vehicles; I recently re-acquired a DotM Cyberverse Ark, and Haywire3 (who turned 6 today!) loves playing with it with his smaller Transformers and Gobots. (he's a kid after his daddy's heart!) Outside of Titans (which are not actually made for the purpose, and also pretty expensive) there's not really much in the way of playsets or vehicles in Transformers toylines these days, even though there is certainly plenty of fictional material to pull from; partly I believe this is because such things would only work well for scales smaller than Deluxe.
TLDR: Deluxe and larger figures as they currently exist make great display pieces, but I feel like there needs to be some form of Core scale figures that are better to play with.