Maybe they can manipulate or crash the XRP market. Who cares? That will only affect people who transact in XRP, not those who transact in other currencies. And the ability to transact in arbitrary currencies is the main selling point of Ripple. The XRP are just an auxiliary tool.
In fact, not only *can* they crash the XRP market, they will, repeatedly. As long as the giveaways continue happening, XRP will be seriously hampered as a store of value. This is one of the reasons the pre-issue doesn't bother me too much: unless they're lying about how they plan to distribute XRP, it will be difficult for them to make much money by cashing out. Their stated intentions making Ripple free to use as long as possible for as many people as possible are incompatible with making a quick buck off of the pre-issued XRP.
Not only will the XRP market crash each time there is another dump/giveaway/massive sale by the founders, they have also explicitly stated that the reserve requirements and transaction fees may be lowered in the future, and each time these are lowered the price will drop as well.