Running late on an inspection is easy to shrug off, but it rarely keeps its footing as merely a fine. Your insurance can lose its grip too, exactly when you need it most.
A penalty is the visible cost. Less visible is the effect on your insurance, where cover can be complicated or compromised if the vehicle was not road-legal at the moment it mattered most.
There is inconvenience too: a car flagged as overdue invites scrutiny, and resolving it under time pressure is always harder than handling it calmly on your own schedule.
Seen plainly, the roadworthiness test is less a hurdle placed in your way than a small, regular policy on your own peace of mind.