From my point of view, we can`t get nothing extra or better performance if we put a road cassette in a hard tail mountain bike.
The riding of the hardtail (speak of speed) is not the same like a road bike.
There are some differences between a road cassette and a mtb cassette .
First of all , "The range " the best road cassettes for example in Shimano (105,ultegra or dura ace ) are 10 speed and 11-21 or 23 t., in increments of one to one as Warp says, really this kind of range are very hard to pedal if we don't have the power and the endurance in our legs, is easy for people like Lance, Ivan ,Jan and co.
The 9 or 8 speed road cassettes are in the low level , tiagra or sora , I don't say that this kind of road cassettes are bad , the high level road cassettes are ten speed.
Finally if someone want install a road cassette in a hardtail bike , before to buy the cassette , please see if you freehub is compatible , the drivetrain is HG and the chain is super narrow in case of 9 speed or IG in case of 8 speed.
Years ago , Cannondale put in some mountain bikes, road cassettes.
Regards.
the last biker.