I don't have much to add but I'll chime in anyway...
I've seen both types of fillers, "lazier/cheaper than singles" and "more elaborate than singles". I have some in my chaos units (now gathering dust but that's another story). Marauders have war hounds taking two slots. A chaos warrior unit has a scorpion-man taking four slots - and the whole space too to its very edges. I found it appropriate for that army as it adds...well, chaos. It's especially true because those units consist of vintage metal and I would have run out of unique poses otherwise.
More generally, I think it's fine that armies like savage orcs and beastmen have big guys or other oddball stuff in their units. I can't imagine anyone there calling for tidy ranks of 24 uniform(ed) grunts. For truly ranked armies, replacing four models with two and a rock feels dodgy, though.
The argument about carrying a huge rock around comes up often but I'm not entirely sold on that. After all, every single element in modelled units tends to be static. Why do the models point and shout or blow their horns all day regardless of the situation? Why do they hold or wave their weapons as seen? Everything there represents something that might happen at one single moment. Nothing there is dynamic so why fuss about a rock in particular? Because it's an easy target for a cheap tirade? I appreciate all modelling effort and novelty. Ranting about static elements in a friggin' miniature display is just sad.
What else...LoS...well, it's a persistent issue in all gaming. Some more competitive skirmish games provide "volume profiles" where you effectively are supposed to treat the model as a fixed-height cylinder in close situations. It circumvents all genuine problems and attempts of trickery, but it also slows things down without contributing much to the overall enjoyment. Most people will soon figure out that it's simply easier not to be a dick, solve the 4+/5+ save issue pseudo-randomly whenever it comes up, and spend the fiddling time on playing more games instead.