Ah so do "free texts" in the USA include MMS? That might explain the continued usage. In the UK MMS often aren't included, if anything they're charged stupidly high, so you'd never send a pic via text message.

yes

To give you another perspective, I’m UK based too and use iMessage daily to stay in touch with my family and close friends. Every now and then I’ll have bad signal (or maybe the recipient does? I’m not entirely sure…) and whatever I’m sending will be sent as an MMS. It happens so rarely it doesn’t really register, and I’d wager it costs me less than a quid a year (on EE, but I assume most MMOs have similar MMS pricing).

On most plans in the US, unlimited texting is the only option, and there is no differentiation between SMS and MMS. It's all free with your plan.