Just my 2p:

I also occasionally use a single earpiece on my commute in the curbside ear, that said I live in a suburban area where the roads aren't heavily congested and half of my route involves bridleway and cycle paths.

I generally also listen to spoken word podcasts at a sensible volume, I don't feel like this makes me unaware of my surroundings or inconsiderate, I can still hear other road users and everything going on in the time that I do spend on the roads.

I agree that two earphones blocking your hearing of others around you generally puts you at risk though.

Apologies in advance if this makes me an idiot.