"Sure they are we've seen a couple of Defiant's and Hoods in Star Trek.
And what about the fact that they have been fifteen ships in the Royal Navy to bear the name Enterprise?"
I said names are not usually reused. The fact that Defiant and Hood have been used more than once does not indicate that it is a standard practice.
So far we have a couple of Defiants and Hoods, the Enterprise 6 times (7 if you count the retcon of the last series) and the Yamato which had an "E" after her registry. There might be others but those are the exmples that have come to our minds easily. That's 4 names out of a fleet of thousands of ships - not a regular practice.
Let's see there is also the Excalibur, the Intrepid (presumed to exist due to the Intrepid Class). So that's now at least 6 ships. So it would appear than ships names are re-used often, even in a fleet of thousands or tens of thousands It's fair bet many of those names have been used before. For example look at the Royal Navy as I mentioned earlier the name Enterprise has been used fiften times, I bet I could with not too much difficulty find more examples.