Even if the Jump Drive isn't used, it requires maintenance. Components for machinery decay even just existing. There's a reason why prepping vehicles for long term storage is a thing.
But, again, this gets into the realm of the hypothetical. Traveller's ship's maintenance is, especially in mongoose, pretty abstract and basically a tiny line item in a ship's budget that is handled 'off screen'.
To get back to the original point, CT (where the 'months past maintenance' rule starts) actually assumed it was months past your annual maintenance, because it only tracked the two weeks a year you needed to spend in the shipyard. The monthly maintenance was accumulated towards that big expense. So it had to be 13 months since your last overhaul before you had a penalty rather than the current "oops, didn't pay my pocket change costs last month" like the current rules.
