Yes I think the next scanner made needs to have bluetooth in it, both for file transfer and for audio.
With that said yes, I use a bt dongle from time to time. There are a couple things to be aware of though so here goes. Most of the little bt dongles you see (like for ipods and other music devices) are only for stereo type bt headphones as they transmit using the A2DP format. This is not compatible with 98% of the common bt headsets (or earpiece, like the ones used with your cell phone for making/taking calls). UNLESS your earpiece specifically says it will also play music and/or accepts A2DP bt signals.You will not be able to pair to the this type of bt dongle otherwise. So if you go this route you will need to use bt stereo headphones or make sure your earpiece can accept A2DP signals. They do make stereo headphones that also makes/takes calls for when you want to use them with your cell phone. I use the Motorola Rockr s9-hd for this. They are small and discreet, won't give you that "Princess Leia" geeky look like some oversized bt headphones and they don't sound half bad. You can only pair up one device at a time to the headphones though.
Another route is to use a bt dongle that is made for older phones that do not have bt built-in but they do have a headphone jack. This kind will pair up to your standard run of the mill bt earpiece. They are harder to find now as most all phones now have bt built-in. About the only drawback is you will need an adapter to plug it into your scanner, as the scanner jack is 1/8" and these types of dongles have a 3/32" plug on them. The one I use for this purpose is the Jabra A210. This setup does the best job, very discreet (esp if your scanner is hidden in a pocket) no one is the wiser as to what you're actually listening to plus you've got one ear open for regular conversations/etc. Battery life on the A210 is around 7-8 hours.
I do have one bt earpiece that does play music (mono of course) and takes calls too and that is the Jabra bt-8040. While it offers the best of both worlds in this instance that battery life is not that long when in music mode (3-4 hours). This earpiece also claims to be able to pair up to 2 devices at once (like your bt dongle and your phone) but I have not tried it.
All in all I guess the final thing to say that might come across a little weird to some (depending on your bt setup), is that if your cell phone rings you'll have to answer it by putting it up to your ear while you have a bt earpiece sticking out the other! Some onlookers might be thinking "WTF"?!