I am a Comcast customer and everyone using their "home gateway" (most customers) are providing free, limited bandwidth access to other Internet customers who are within range. As I walk the dogs I find that I am almost continuously connecting to one house or another. Same when driving around. My phone checks for adequate performance when connecting, but hangs onto the connection even though throughput is about 10 bytes per second. and, even when the signal is lost, it takes about 20 seconds before giving up.
I am forced to turn off WiFi to listen to stream or even get a sports score update. Then I get home and forget to turn on the WiFi and THAT is when I waste a bunch of my data allowance.
As to not noticing the switch-over, streaming apps do have a noticeable "glitch" when moving from WiFi to LTE. It's quite likely that you would notice. The app has to detect the change, open a connection via the LTE address and start the new stream running before the buffer empties... when the low quality can only be detected until the buffer is already shrinking rapidly. There are several tricks apps can use to minimize the delay, but I have yet to find one that does the switch transparently on audio, let alone video.