Unlike other keyboard layouts, reducing same-finger was a high priority in the Colemak design.
In fact, Colemak wouldn't exist without this consideration. While designing Colemak, I've excluded all digraphs that caused same-finger beyond a certain threshold. Which meant that in certain positions, only a very small percentage of keys needed to be considered. In many cases, while taking into account other factors, it meant that only one letter would be a good fit in a certain key location.

* In QWERTY, for example a lot of people learn "typing tricks" to speed up typing, e.g. for the "ED" digraph, they'll type the E with the middle finger, and the D with the index finger. This increasing the learning time, complicates the typing process, and often encourages bad finger/wrist postures and bad typing habits in general.
* Same finger slows down typing. In order to achieve a high typing speed and high accuracy, you need to maintain a steady typing rhythm. Same finger interrupts the typing stream. Interrupting stream increases the error rate. When errors are made, the typing stream is interrupted even further. While it may not be very apparent initially, it becomes more and more of an issue as you develop your typing speed and skills.
* Same finger magnifies the strain and effort put on the finger, e.g. try to type qzqzqzq...