A friend considered it, she is a vet nurse and decided it wouldn't be fair for the mare to carry the extra weight on feet that were already tender.
Hope that helps
Think carefully about this. being in-foal can actually cause laminitis in suscepticle mares. Also you need to consider all the usual breeding things, how good is the mare, what are you hoping for (eg waht use for the foal)and what will you do with the foal, sell it, keep it? Many more other things to consider too.
You might like to post this in the breeding section.