A lot of the skills you learn as an FI can be directly related to the job of being an airline pilot.
There is an awful lot more to being a pilot than just the basic handling skills of flying on instruments.
As an FI you learn how to work wth people in the cockpit, you gain confidence, you get scared, but most of all you learn decision making skills.
Being an FI helped me massively when I got my first airline job. You can even now see the difference between those straight out of school and those who've done a bit of instructing. The handling skills might not be that different, but there is a huge difference in their thinking and let's face it, that's what flying an airliner is all about nowadays.
So saying an FI has no "relevant" experience is totally wrong. Certainly on a bad night, I'd rather be sat next to an ex-instructor rather than someone straight out of school who has never really been in command of an aircraft in real terms.
Airlines currently seem to love cadets that they can mould in their own image, but long term, I personally think that isn't going to be great for the industry.
It is unfashionable now to work your way up in this business and everyone is in a rush to fly shiny jets, paying for ratings etc.
Who would be the better pilot really? the person who has instructed, flown IFR in marginal machines, turboprops and then gets on a jet or the person who walked out of Oxford paid for a rating and then went straight in as an F/O on a 737 or airbus?
I know who I'd prefer to sit next to, the old school method pilot would probably have an awful lot better line in stories as well and when you're sat next to someone for 10 hours a day, that is one thing you'd be thankful for!