Well, first of all, I think you nailed it with the word "clueless". They are. I think most of us are clueless about other people's lives and struggles, though I would hope that most of us, MOST of the time, wouldn't say something hurtful in our cluelessness.

How to describe MS to an "outsider" has always been the $64,000 question, hasn't it? Multiply that by the factor of every MSer being different, and every outsider being different in comprehension...exponential.

A possible return email might begin something like: "I'm sure you didn't intend your comment to be hurtful to me, but I was taken aback by the thought that someone might think that a person with an incurable disabling disease could somehow be using it to get a free ride..." ..."If you'd really like to know how I got on that 'gravy train', I'd be glad to share some medical information about Multiple Sclerosis so you can decide for yourself if what I had to pay for the ticket is worth it...."