1) If you match 49 other people in your country and can explain your whereabouts, as can 48 others, the guilty party is going to stand out. If you can't and someone else can't then theres doubt. Combined with other evidence it should be pretty safe.
So lets assume you unfortunately share the same DNA as a serial offender.
And, as I do, you have difficulty remembering what you were doing more than a couple of days ago;
Or you don't use Facebook to document every minute detail of your boring life;
Or let's say you went out for a few beers after work and the guys said lets got to a strip club. You went but you didn't want to tell your wife.
And then the police come knocking on your door asking you to explain where you were at a certain time and date, which may be many days/weeks/months ago.
And then, lo, the serial offender strikes again. And the police come knocking again...
How many times do you think having the police questioning you is reasonable?
How many times do you think having to 'pop into the police station to make a statement' is reasonable?
What happens when you can't/don't want to have to account for every minute of your life?
How many times do you think that this would happen before the police start to draw their own conclusions?
OK, so most of this is circumstantial but I for one don't want to have to justify my life to the authorities just because I happen to be a match to someone else on a database. Even if this happened once, that's once too often and I for one would never 'pop into a police station' without a solicitor present.