The two big downsides I see are that:

First, it needs to be filled right before whatever disaster causes you to lose city/well water.

Second, once it's filled, you can't really move it. So, you also lose the ability to use your tub for anything else (like bathing with the water in your water BOB).

While I guess it might be worth having for someone that lives in an apartment and can't normally store much water, but if I have a house with some storage space I'd rather store water another way. Better to have plenty of water stored in advance than having to worry about filling your tub right before an emergency hits.

Very true. Probably best for an apartment dweller that can't bug out but has some advanced notice.

I note that when we had a power outage that covered all of Southern California about six months ago, we still had water pressure because the pumps were on emergency power. However, they told us that the pumps wouldn't be on indefinitely.

So it some cases, you might well have water for stockpiling for a short time so such a bag would be useful in such situations.

Oh...that's a real turn-off for a product like this. Basically turns it into more of a disposable item.

What about it made it impossible to empty completely? Maybe the position of the spigot in relation to the "bottom" edge of the bladder?