While it may make sense to you, it does seem to me that by far the vast majority of people will place a contact's mobile number under the lable 'Mobile'. Most people use the term 'Home' to mean a land line (Yes, some people still have those). So, while I do understand your frustration, for most of us the fact that WP defaults to texting to a number marked as 'Mobile' (since to my knowledge only mobile phones can send/receive text messages) does make sense.
Moreover a user of TellMe (which I personally use extensively) can say 'Text John' and WP immediately knows that, of all the phone numbers I have included in John's contact information, my intention is to use the one which I have designated as 'Mobile'. Of course the argument could be made that TellMe could allow texting to target numbers just as it does for calling (e.g. being able to say 'Text John Mobile' similarly to how one can currently say, 'Call John Mobile'). But omitting 'Mobile' from the command to TellMe should still default to texting to the number designated as 'Mobile'.
I personally think that it's a reasonable implementation, but again I have designated all my contact's mobile phone numbers as 'Mobile', so it all works as expected for me. I wouldn't want to have to edit 800 contacts, but then again I only actually regularly text with about 15 or so of my contacts.