I would think if the mare is lame due to a congenital abnormality, then it would be tantamount to insanity to use her as a broodmare and pass along the same genetic possibilities to her foal/future horses in the line, even if it did skip the initial foal...
IF she is dealing with an injury based in no manner upon any weakness inherent to her, but rather, she (for example) suffered a bad fall or random accident, and has valuable genetics to pass along, AND she can comfortably foal, I say give her the opportunity to be bred, (to a sire of value as well, of course).
Just my inexperienced point of view... :0)
"The argument from intimidation shows nothing but intellectual impotence." Ayn Rand