not necessarily that not useful = bad, its just that adding a feature to the game that doesn't really create any depth is somewhat useless, panning/sluicing seemed like an acceptable way to add much depth to the game using rubble, roads and blocking water just don't seem like significant enough reasons to add it, neither does slowing down mining as that can be accomplished in better ways.
The more I read, the more I think that depth means something very different to you than it does for me and people like NW Kohaku. To me, a logistics problem that forces you to rethink how you organize your labor force, supply change, and even rework your basic architecture adds a
lot of depth. Making the rubble processable through sluicing to get a small amount of a useful mineral isn't necessarily a bad thing, but adds very little depth.
I think the value of rubble would be similar to real life: it is basically worthless, but you have a lot and have to do something with it, so use it as filler instead of using a real resource. Fill in walls with it rather than using more expensive stones, use it to raise up a portion of land, fill in a valley to make a dam, mix it with cement to stretch your supply, etc. The only use that exploits any real value in the thing itself is sluicing, which only makes sense if it is the rubble generated from ore mining. Sluicing granite rubble is unlikely to give you anything of value.
I like the idea of ordinary stone producing generic rubble, with no tracking of source stone type, while ore and flux stone produce rubble of specific ore or flux type, and the hematite rubble is what gets smelted, rather than hematite boulders. This might cut down on using economic stones as high value stonecrafts. Limonite is valuable because it makes iron, not because things made from limonite are especially attractive. Ore rubble could be stored for use in several ways: could be put in bins and stored in stockpiles, it could be piled up in a corner, and if the sand-flow suggestion is implemented, it could even be stored in big hoppers with an opening at the bottom that a small quantity flows out of to form a ramp. If ore produces or rubble, it would be very annoying if default rubble hauling behavior caused your valuable ore to get dumped in the tailings heap. It could be excluded from default hauling, and just be left in place until specifically moved. If rubble blocks traffic or digging, this would be very annoying, and I think it might be annoying to set up separate storage of the rubble of different ores, so it might be better to leave ores boulder based.
A rubble based system would be different from just slowing mining because there isn't currently a real motivation to clear the rubble. It doesn't hamper movement, it doesn't annoy dwarves, and it doesn't prevent the tile from being used as a stockpile. It's just kinda ugly (but hideable). If you were actually encouraged to remove it before work could continue it would add a lot of depth, compared to the current system, which doesn't really have any.
One way to make rubble less annoying for players would be to make a rubble tile passable with a speed penalty. There would still be a strong incentive to clear the rubble, but if there's a sudden hauler meltdown, and everyone is busy bringing things to the depot or something, it doesn't put a complete stop to mining work, just slows it until someone can deal with the problem. This could be avoided by having dedicated stone haulers.
Another way that rubble disposal would add depth is making contact with the surface more essential. Currently the only real use for the surface is wood, which is plentiful below ground, and caravans and migrants, which can usually be ignored. If you had to do something with your rubble, you would need to have some surface access, unless you arranged an alternate solution. It could be atomsmashed, but if rubble were added specifically as a logistical challenge, it might be treated like a large creature, and prevent lowering of the bridge. It could be dumped in the caverns, but if it were made to stack and flow, as per the sand suggestion, it would quickly pile up and clog the dump chute. It could be dumped into the magma sea, which might be a good permanent solution, unless rubble turned into an equivalent volume of magma, in which case you might risk flooding your fortress with magma (unless magma flows off map edges in the magma sea, does anyone know about this?).
I personally do not understand the "lets try to recreate all the potentially annoying parts of rubble without the rubble". Slowing down mining adds difficulty and tedium without depth. Requiring stone boulders to be removed adds difficulty and tedium with only a tiny amount of depth. Requiring space filling rubble to be removed adds difficulty and a small quantity of tedium (unless it's implemented well), as well as a huge amount of depth. To me, slowing mining is just a side effect. It's the rubble pile (and object piling in general), and how to deal with it that are the whole point.
EDIT: Many players
do want support struts, ventilation, and illumination to be necessary. A common feature of the proposals for all of these things, as well as for rubble is that it would not be necessary for initial diggings, and it would only be as you expand your operations that things would start to become complicated. No matter how many times people say "you just want us to have to set up rubble management and minecart systems before we can even start digging", that doesn't seem to be what
anyone is proposing, and it's disingenuous to pretend otherwise.