Why big output buffer in Furnaces?

[jamiechi1]

Because in the early game, you can setup furnaces to smelt without electricity, using burner miners and burner inserters. It would be incredibly inconvenient that they would only stack to 4 or 5 in the furnaces, meaning you would have to keep emptying them very regularly. ...

I disagree.
Actually it would not be inconvenient. Thats what the boxes are for. Place an inserter and a box to put stuff in on the output of anything that produces other things.

What this does is greatly increase the inconvenience of requiring the use of circuits later to fix the issue.
The early game is so short in the beginning, unless you have mods that extend the early game, restricting the items in the output from 2 to 5 is not an issue.

[FuryoftheStars]

Just to throw my 2 cents in here, I prefer them the way they are. I typically like early game being able to do direct miner-to-smelter, or even later, once I start belting, to be able to omit the extra work of output inserters (and their fuel) and storage. The fact that later once it's no longer needed is still there and eats up a lot of resources doesn't bother me. I mean, once a setup has been going a while and maxes out, there's functionally no difference between a max of 100 or 5. It's just an in-built buffer if you have a sudden rush on demand. And what else are you going to use the ores on?

Steal production is the only place where I can think of that you'd want to limit initial intake until you had a surplus of resources. And that's not that hard to deal with for the time being until it, too, maxes out, or you get circuits researched and in place.