None of us are into boating to save money.
I don't raise my sails to save money on fuel. I do it to add stability. I do it it to reduce or cut out the engine noise. And I do it for fun. Especially on a long crossing when it might get a bit monotonous just pointing in the right direction, and waiting until you get there. You can play around with the shape to get maximum efficiency with every wind change, or just leave them do their thing and relax with a more stable and quiet ride.
I've never raised them for backup propulsion, but I enjoy the fact that they are there if ever required.
Regarding cost of sail rig maintenance; In my experience - it is minimal. Over the last two years, from memory I have spent a grand total of about $30 on maintenance and upkeep. (I've dropped a couple shackles into the water) That's it. Compare that to a second engine.
A motorsailer has much less stress on the sails and rigging in comparison to a "proper" sail boat, due to the lower sail area to weight ratio. This results in longer lasting equipment.
For those who want a turn key method of transportation on water with minimal fuss- a motorsailer is not for you.
Nor is it for those who want the most efficient wind driven machine on the water.
It is for those who enjoy comfort, like to tinker, and stay in touch with their environment while enjoying their boating experience.