The best reason not to fund Planned Parenthood is never mentioned in the newspapers. (Could it also have been overlooked in the briefs?)
Numerous undercover investigations have shown that Planned Parenthood prefers money to the interests of women. But even without that evidence, it would be unwise to entrust pregnancy prevention and pregnancy counseling to Planned Parenthood as long as it is profiting from abortion. No organization that is supposed to prevent pregnancy, or counsel pregnant clients on their options, should be making money from a particular post-pregnancy outcome, i.e. abortion. Fair and neutral pregnancy prevention and counseling will be much more likely with organizations or agencies that do not have such a built-in conflict of interest. (German constitutional law, for example, requires the separation of abortion counseling from abortion provision, and South Dakota has wisely moved in this direction recently.)
The above rationale should easily withstand both constitutional and regulatory challenges, for it focuses solely on benefits to clients, not on the pros or cons of abortion.