Op I think it depends on the outlaw you are messaging.
My sil SMS my parents for babysitting, dinner arrangements and they find it really frustrating ... They are both technologically challenged (my dad can SMS now but can't figure out how to put spaces between words), have crappy phones, messages seem to take hours to be actually received.
One day sil forgot about "special persons" day at school and sent my parents a message to please go ... It didn't come through till the time they were supposed to be at school (they jumped in the car).
The weird thing is my folks are great on the phone ... You don't have to have a long conversation, just tell them what you need it takes a minute!
The other thing sil does is send them messages on fb ... They rarely check it and still can't figure out the diff between a pm, status etx. So for my nieces birthday she sent a message to everyone about dinner - my folks never got it. Or same nieces birthday ... My mum called sil to see if there is anything she would like - sil sends her a link on fb.
I've spoken to her, my brother has spoken to her but she still does it!!! So its now at the point where it is rude.
Sounds like your mil is better with technology than my folks (anyone pretty much is!!). So maybe just ask her if she prefers a call and your reasons for SMS.
Personally I love SMS for all the reasons everyone has listed above but I'm conscious of who I am using it with ... So just because the technology is available doesn't mean it's a good fit for everyone