I was reading the spec again and noticed one of the criteria isn't met in order for the transceiver to be reused:
[x]sender's track is null
[x]sender's kind matches
[x]transceiver is not stopped
[]sender has never been used to send

In the case this isn't a bug. I guess my main concern is that the SDP keeps growing every time you call addTrack /removeTrack. Is this an issue?

hbos@: can you take a look?

As noted in #1, one of the criteria of reusing a transceiver is "The sender has never been used to send."
The list of transceivers can grow, but it cannot shrink. When RTCRtpTransceiver.stop() is shipped, you will be able to clean up the SDP a bit, but getTransceivers().length will forever grow.

If you want to reuse transceivers you should make use of the RTCRtpTransceiver.direction attribute (changing the direction will trigger onnegotiationneeded). With RTCRtpSender.replaceTrack() you can seamlessly change which track to send (or stop sending if you set it to null) which doesn't even require renegotiation, which may be a good idea if you want to switch camera or mute the stream.

If you want to get into the "transceiver mindset" you might as well start using addTransceiver() instead of addTrack() to make it more clear that you are in fact adding transceivers when you're doing this.

Not adding an unnecessary number of transceivers can be a good idea, in case resources are wasted on encoders and decoders that aren't used.