because they are all mirrors of the ibiblio repository and don't get updated at the same times. the mirrors are in different parts of the world and some are administered by volunteers who have to fit in maintenance with their jobs.
with more than one repo enabled, your packages list can become scrambled with incorrect entries, leading to mis-installation.
Maybe Synaptic should then dis-allow the user to select more than one??
I don't know ever thing, but here is one explanation as to why Synaptic can use an other repository mirror as well.
Most of us are from Native English speaking countries, USA/CAN/UK/AU/etc. and for us we should only use one repo mirror at a time.
BUT there are some users from other countries who use PCLinux (even MiniMe) and often in these countries mirrors their PCLinuxOS communities have created additional packages that are often related to non-English packages or things like that. The Thailand community does just that and these packages are only of use to Thai speakers, it is not quite the same pclinuxos repo path for these additional packages so they have to enable two repository mirror paths for it to work.
So unless your worlds/country community does some thing special like this, it is recommended to only use one repo mirror at a time.