Once again Miller came into a LOOGY situation with men on and gave up an extra-base hit, allowing both of Harper's runners to score. So far this season Harper has been one of the most effective relievers in baseball at not allowing inherited runners to score, allowing only three of the 18 he has dealt with.
I don't know exactly how it is tracked but Baseball Prospectus has a stat called INR (Inherited Runners Prevented). I would guess that it weights the difficulty of the situation (Man on first with 2 outs is much easier to prevent from scoring than man on third with no out, for example) then assigns a value to each inherited runner pevented or allowed to score, producing the overall stat.
Top 4 INRs in baseball
Danys Baez - 5.3 (1/13 scored)
Brendan Donnelly - 4.8 (0/9 scored)
Julian Tavarez - 4.7 (1/13 scored)
Travis Harper - 4.5 (3/18 scored)
Conversely, there's a stat called BQR (Bequeathed Runners Prevented), tracking which pitchers have been let down by their teammates allowing too many of their runners to score. Harper has the second worst BQR in the majors at -1.7, having an astounding 7 of his 11 bequeathed runners come in to score after another pitcher came in.
There's a final stat to this report called FRA (Fair Run Average). What is does is take a pitcher's Run Average (not ERA, this includes "unearned" runs) and adjusts it based on how good he is at preventing inherited runners from scoring and how bad his teammates have been at allowing his bequeathed runners to score. Harper's FRA is only 3.93, reflecting his excellence at handling inherited runners. This isn't anything unusual for him though, he was fifth in the majors in INR last season.
Who is the worst in the majors so for in 2005 at letting inherited runners score? Trever Miller. I was surprised though looking at last year's stats, he had a very respectable 5.1 INR. I guess it just seems like he has trouble with inherited runners, skewing my perception. That of course is why we have stats, so we aren't dependent on personal, very subjective recollections of performance.