The idea of a filler map (i.e. dosn't particularly stand out) seems a tad unfair for 'conventional' maps, since it just comes down to whether or not the player connects with the map, which itself comes down to the stylistic preferences of the creator...
Anyway, filler maps help, but they're by no means required to fill out a cohesive map lineup. Best two examples off the top of my head are Fractured Worlds and Sunlust - they both avoid filler in their own ways, and both were amazing, with every detail of each map burned into my memory.
SL manages to avoid being exhausting by not being in any rush to overwhelm the player (contrary to its reputation). Aside from the first, every map brings new and interesting ideas to the table, but until the late maps, they don't pile on any more than necessary. As a result the maps, while hard, are never unduely taxing to play, demanding 𝘴𝘰𝘮𝘦 level of attention, while doing what it can to 𝘦𝘢𝘳𝘯 said attention.
( I think this is where magnum opus syndrome comes from - demanding attention without earning it - but that's all tangential)
FW is different - rather than not having filler maps, it is almost devoid of filler fights. The effect is exhausting when starting out, but once you get going, it is exhiliarating. It avoids the phenomenon of every map competing for attention by continuously escalating the stakes, each map more dramatic or intense than the last. (map06 is the exception to this, but makes up for it with the single best secret area ever made)
