Well this is getting more and more interesting by the day.
I have to agree with the majority here, sills and frames surely must consitiute a "clean"
The MOT scenario I personally think doesn't add up. Reasons? Well for 1 the Garage has to pay the commission on the MOT certificate and the extras that are not covered on a service will cost more in parts and labour and will, I hope, take more than a few seconds to complete.
In the end it really depends on how you want to "sell" your service to the customer. I like to think that when I have completed a job the windows, frames, sills and even the doors look good. I like to believe my customer is happy and wouldn't have any hesitation in recomending our services to her friends.
If you have just cleaned her house and ONLY cleaned the glass and NOT the frames I wonder what her friends would think of a) the finished result and b) the window cleaner, when she looks at lovely clean glass and minging frames covered in dirt and algea.
Surely her question has to be "don't they clean your frames?"
Answer - "no, thats extra!"
REPLY - "Really? my window cleaner cleans the whole lot and still charges me the same as you!"
Customer stands and thinks "yeah, but my glass is clean

Like you have said in a previous post, you have done this for 25 years and for some of the big boys so it obviously works!
I would just be worried that another window cleaner offering to do the complete service at the same cost would surely make even my most loyal customers question what they are paying for?