Unequal splits are an invitation to resentment and the degraded communication and problem solving that comes with resentment. If you've done a bunch of startups, you've probably had the experience of having a harrowing argument with cofounders, and the realization of how easily your company can disintegrate based on nothing more than silly interpersonal conflicts. Some people, myself included, take from this the lesson that you should rig your startup to minimize these issues as much as possible.

Another reason to avoid unequal splits is that they often presume you can see into the future. Who's to say that whatever you did to "earn" an extra 10% of the company isn't going to be more than offset by something your cofounder does later?