"Kelly is a veteran point guard that can also shoot and play off guard position. This frees up Cappie to do what she does best, ‘shoot and score’ and not have to worry about point guard duties," said Whisenant. "She is tough and athletic and will fit our hard-nosed defensive style very well. Leilani and Kelly make us very stable at this very important position."
It's really that simple.
The Liberty were notably less efficient offensively when Cappie Pondexter had "to worry about point guard duties" last season in part because in trying to initiate the offense she took herself out of scoring position or in trying to put herself in scoring position she failed to initiate the offense. Miller just gives them an option when Leilani Mitchell goes to the bench that allows them to keep Pondexter in scoring position.
Although Miller has declined significantly since her days with the Phoenix Mercury (2006-08), she still had a positive pure point rating and has remained a strong 3-point shooter with a 44.1% percentage from beyond the arc last year with the Washington Mystics, second best of her career.
For the Mystics, it does look like they might want to beef up their point guard position, but with two first round draft picks, Jasmine Thomas, and Noelle Quinn - who has been an efficient ball handler, as a lead and secondary- they're not in a dire situation either and letting an aging player go is not a bad thing.
Perhaps as an aside, implicitly stated here is that 2012 Liberty draft pick Sydney Colson is not a major part of their plans to make the Liberty stable at the point guard position.