It seems reasonable to me.

Getting the Starship back is of huge value of them, and one of the main things that would stop that would be to lose enough engines that you don't make it to orbit. There's some payload number that lets you make it to orbit with, say, 2 engines failing to start - and beyond that, you just lose control of the vehicle all the time so it doesn't matter.

Assuming the payload is low enough value, it makes sense to fill it up as far as you can without lowering the probability of success unrealistically.

> the loss of ~$10-15M worth of launch capacity by letting it go unused

What is the production rate of these birds?

If you have a 50/50 chance of success now and a 25% chance a month later, and you can't replace the lost birds in production in that interval, it makes sense to hedge irrespective of production cost.