Product review (3 stars): This is a poor design by the manufacturer and there's really nothing that ...
In 14 years, I've had to change the rack 4 times.  The middle slots are the ones where the plastic always becomes brittle and eventually becomes a hole.  This is because the heating element is directly underneath and the plastic is exposed to the heat here more than the other slots.  That's why you'll find that it's the two middle slots that wear away the fastest.
This is a poor design by the manufacturer and there's really nothing that can be done about this other than to keep changing the racks every few years.