Paul;
I know of a TT newcomer this year who would have preferred to race in the Manx first, but the option to do so wasn't explained to him. By the time he was made aware of the fact that it would have done him no harm to learn the ropes at the Manx, and probably move on to the TT the following year with a newcomers MGP race win at the top of his CV, it was too late. His sponsors had booked boats and accommodation, and weren't prepared to change things.
It should be up to the rider to decide what he wishes to do, and all options clearly spelt out to him.