The pokerus CANNOT spread inside your PC. By putting it in the PC you can preserve the pokerus, quite literally forever. Once a Pokemon is infected, it has a limited time (1 day IIRC) that it can infect other Pokes in your PARTY. Once that time is up, it will still have the virus, but it will no longer be contageous. That is why you keep the Poke in your PC and then bring it out with 5 other unaffected Pokes to spread it to them.
Pokerus really is a big deal, if you are familiar with EV training. It can cut your training time by 75%! That is huge. For instance, if your Pokemon battles against a Clefable, without pokerus or Macho Brace, it receives 3 HP EV points. If it battles with the Macho Brace, it gets 6 HP EV points. If it battles with the MB and pokerus, it gets 12 HP EV points. That means it will take 84 battles in order to max out the HP EV stat (252 points) without the MB and pokerus, compared to 42 battles with the MB, compared to only 21 battles with both MB and pokerus, or SIXTY-THREE less battles. Huge.