@Bob - I use the inReach very much as you have described. I don't always know when I will stay longer or if I plan to bail early, so it's nice to have an option to let others know your plans or call for a ride at a new pickup point. Being able to get confirmation is a big plus to me. I have also used it to let my backpacking partner know where will camp for the night so they can join me on the trail. If you use the Earthmate App on a smart phone, they have a feature that allows you to pick a reference point on the map (a point that is not your current location) and send the reference point coordinates in an email. This is a great feature for identifying a unexpected pickup point.
I also don't think you need to abbreviate your messages (other than for convenience... typing on that on-screen keyboard is painfully slow!). You can send up to 160 characters and you are charged per message, regardless of the number of characters you type.
From what I've read, when you send an SOS message with the inReach, it will prompt you for confirmation that you really want to send an emergency message. If yes, you will get a pre-defined message on your screen that requests help and you can alter the messsage however you like. When you hit "send" it will count down for 20 seconds before sending. This is presumably to allow you to hit cancel. Once the message is sent, the unit's tracking is locked on (you can't turn it off). When your message is received, you will be sent a message confirming that your request was received and help is on the way. That's a really nice feature that Spots and PLB's don't have.
As for the service plans, you can suspend your service when you are not using the device and it only costs you $4 per month to keep your account active. This feature makes the plans affordable in IMO. In Canada, we have a different service plan than in the USA. We get a pay-as-you go service. It's not well advertised, but if I switch to the Expedition Plan for a week and then suspend my service, I am only charged for the week of usage (they pro-rate the monthly cost). I have just tried this and haven't seen the bill yet, so we'll see if it's as good as it sounds.