The reason to use the metric system is to make unit conversion easy. There are two contexts where this comes in handy: scientific research and money. And indeed, we do use the metric system in that context just like everyone else. We also use 5, 10, and 100-based denominations in our currency.
However, there's no reason to do unit conversion at other times. Maybe this wasn't true in the 1970s when the UK switched, but these days, with embedded electronics installed in everything, all tricky arithmetic has been automated out of our day-to-day lives. (For example, this is why unit conversion is no longer important in navigation-- we have GPSs now.) Unlike a lot of people here, I don't reflexively see this as a bad thing.
Now, take a moment and think about how much work it would be to switch an entire country over to the metric system, especially one the size of the US. Think about the expense involved in replacing all the relevant signage-- both public expense and private expense. Think about the expense (and possibly even loss of life) originating from the confusion (Oh hey, the speed limit is 100...)
I don't think you can really justify that, just so you can figure out that if your house was a fishtank you'd need 6 million liters of water.