Why shouldn't a judge on the United States Supreme Court use her personal knowlege as gained by life experience as one component of what goes into making decisions?
Conservatives claim to be 'strict constructionists,' regarding the Constitution and claim that judges should relate everything back to it (well, except for the fourth amendment, the fifth if accused of a drug crime and the sixth regarding Jose Padilla, and portions of the first, also not Article 4, Section 1 as relates to marriages performed in New England states.) They blithely preach the Constitutional gospel like it is the answer to all life's questions, even though the founding fathers themselves took pains to make it clear that it only served as a guideline. For example, conservatives love to remind you that there is no explicit right to privacy in the Constitution. There doesn't have to be, thanks to the ninth amendment which explicitly states that the fact that a right is not enumerated in the Constitution does NOT construe that there is no such right. It's up to judges then to enumerate what additional rights the ninth amendment refers to and more than 90 years ago they decided that privacy was one of those rights.
Obviously to conservatives though these apparent contradictions of their dogma with the Constitution are mere inconveniences. According to them, Ben Franklin and John Adams must have agreed (and agreed with their view) on medicinal marijuana and funding for English learner classes, it's in there if you look hard enough. And then see what you want to see.
Only problem is, if it was that easy the case wouldn't make it to the Supreme Court in the first place. If the Constitution clearly spelled it out then you wouldn't even need judges at all, just a bubble sheet with questions about the case on it and a computerized flowchart.
The cases that the SCOTUS even accepts have been appealed all the way up through the process precisely because there are often contradictory Constitutional provisions, clauses or amendment which can be construed as favorable to either side. In other cases there is simply no Constitutional precedent at all.
In these cases, why isn't it apropos for a judge to use all sources available to arrive at a decision, including her own experiences? The Constitution may not provide a clear enough answer to a problem that was not even conceived of two centuries ago, and in that case the judge has to have something to go on, be it her own judgements, international law or whatever else. So why make it a bad thing to use her best judgement?