Actually the way you use it as a liner for batch sparging in a cooler would work great for me. Do they make them for round coolers? It seems it would be easier for me to just pull the bag, give some grains to the chickens and the rest to the compost. Its a piano (Edit PIA: damn autofill) to dump from the cooler with a false bottom.
As far as biab goes I love it for 2.5 gallon batches on weeknights in the winter in my kitchen and its made me a better brewer because I have been able to do way more batches. That being said, if I have the time and the weather is OK I prefer batch sparging outside and I wouldn't want to do 5 gal biab.
I use a bag from bagbrewer.com in my 5-gallon round cooler and it works great. He will make it to any measurement you specify. I had him make it wide enough to use in my kettle in case I ever wanted to use it on the stove as a traditional BIAB, but high enough to line my 5-gallon cooler. My procedure is to do a full-volume mash in the cooler, sort of a hybrid between BIAB and more typical cooler mash. The cooler helps hold mash temps for me better than keeping it on the stove in my kettle.
I agree that BIAB would probably be unwieldy for larger batch sizes, but for my purposes it suits me just fine.