This facility was flagged up at a meeting of the UK Search & Rescue Inland Consultative Committee that I attended on behalf of the LDWA. I believe the idea was publicised in a Fell & Rock Climbing Club news item. You are right to point out the limitations of this service, which is primarily intended to assist people who are hard of hearing or who have a speech impediment. Someone recognised that it may have benefits for the outdoor community but this was not what it was designed for. It is more of an "if all else fails" facility.

This item in the checkpoint section of Strider No. 119 suggested registering to be able to use text messaging in case of emergency in a hill/mountain area where the mobile signal is so poor only texting is possible. A good idea if you spend time in such places however the item indicated more details could be found at www.emergencysms.co.uk. I had a look at the site and as of 29/11/2011 it had little to add and no mention was made of hill/mountain use so I made an enquiry about that. Plus I have been told by many mobile users that they have found themselves connected to French mobile networks while walking in the Purbecks on the Dorset coast) so I asked about that as well. Below is an extract from the response.

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While the emergencySMS service is primarily designed for people who are deaf and
speech-impaired there are other situations where it could be used. It is likely that
you will need to register your mobile phone number for the service but that there
will not be any restrictions on who can register.
It is important that people understand the limitations of the service. Firstly as it
uses SMS it is not guaranteed, and secondly you need to send a number of messages
before help is sent. The last point is very important because people imagine that if
one SMS message gets through help will be on its way. This is not true as the
emergency services always asking questions about where you are and the problem. If
the emergency services do not receive replies to their questions no action will be
taken. This exchange of SMS messages means that making a voice 999 call will always
be quicker.
If you are in the mountains where the mobile coverage is patchy and there is a
likelihood that only one message will get through I would always suggest sending an
SMS message to someone who knows you, your route, and can contact 999 on your
behalf.
If you do publicise the service please make sure that people understand the
limitations and that they visit www.emergencySMS.org.uk for more information.
An SMS sent from outside the UK by a mobile phone that is registered with the eSMS
service should be rejected by their mobile network. There are no facilities to
support mobile phones that are roaming outside the UK.

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So if you have to use it you need to remain where you can reply which could be tricky if you have had to move from the incident spot to get any signal at all. If your phone can only make contact outside the UK mobile networks your message will not get through.