Here's a useful link:
http://www.protegefaq.net/tsb/09-002-03.html
It mentions the maximum weight supported by the rack, 75kg.
DESCRIPTION
If a vehicle was factory equipped with the roof rack side rails, the cross bars can be removed or installed. Please follow the instructions below for removal and installation of the cross bars.
NOTE: The roof rack must be equipped from the factory. The roof rack cannot be installed as an accessory. Mid year 2003 the roof rack cross bars will no longer be installed on the vehicle at the port, they will be shipped in the trunk of the vehicle, to be installed at the "customers discretion".
CAUTION:
All warnings and cautions apply to technicians and customers using these instructions.
This installation requires two persons to avoid damaging the vehicle.
Wear work gloves to prevent hand injuries from sharp edges or tools.
Tighten all bolts evenly from side to side to prevent damage to the roof rack or leaning causing distortion of the rack and making the rack crooked.
Carefully observe all torque specifications.
The maximum load capacity of the roof rack is 75 kg (165 lb) and must be evenly distributed across the entire roof.
The roof rack is not designed for direct loading, special accessory attachments are available from your local Mazda dealer.
When using an accessory kit, always follow all instructions included with the accessory kit.
Because bolts and screws may loosen in use, periodically check torque and tightness of all bolts and screws every 100 km (62 miles).
NOTE: The cross bars can only be installed in a vehicle that came from the factory with the side rails installed.