Well, if your startup starts to generate significant cash, giving up even a fraction of a percentage of equity as preferred stock (with typical preferred stock rights) is the difference between paying yourself a startup founder's salary and paying yourself as much as you possibly can. In other words, you're shutting the door on a very common way of getting wealthy, the one the vast majority of successful small business owners use.

Certain things about taking investment are binary - percentage ownership is nothing, investor's rights are everything.