Dad's input here, and one who is doing the loft thing now.
Is this a "forever home"? or at least going to see out the children-leaving-home type forever? If so, I think the decision is weighted far more in favour of your family's needs (as oppose to thinking as much or more about the consequences for resale).
A family bathroom and an ensuite on the master is more than enough for a family of four. So if the intention is for it to be an office and hobby room, but not a bedroom, then I don't think a bathroom is worth the initial expense. or work and expense to clean/maintain.
However, if you actually want it as a master bedroom, or if you think there's a high chance of selling in the next 5-8 years, then yes I would probably put a bathroom on.
If you haven't actually reached a firm agreement between you as to what you're really trying to achieve, then it's going to be hard to see the decision the same way.
If you have a big bedroom with an ensuite already, I can easily see why your husband has no interest in creating a new en-suite master bedroom in the loft. Loft rooms are hot and noisy, not to mention the often disappointing shower pressure!