Having built one of these services
Having actually built a 'excuses' service which you could setup from a web page to text you at a predetermined time with a predetermined header I'd have to agree with ICSTIS. The service would send e.g.
msg: Hi son, come quickly, your Nan's cat is stuck up the stree again
Which is all perfectly fun when you're on a date with a chick you don't want to stand up but want an excuse to exit from if it's going badly.
But consider someone who knows your mobile number sending...
msg: Hi son, come quickly, your Nan's been hit by a car
Now it's not fun but all you know is the text came from 'Dad' .. there's no shortcode in the header to link it to a serviceprovider and mobile networks are utterly shite/incompetent/unwilling when it comes to resolving a header to a shortcode. In other words you'll never know who sent you that unpleasant message.
In general, how many time have you received a WAP Push (service message) or a text from 'notanumber' and not been able to find out where it came from because the network can't resolve it to a shortcode?