its up to how long will u keep teh camera or how often u take it for shooting..
if u think ur house or room is humid and lots of dust (like me) u better get 1..
btw im live/work at genting...so a dry box is necessary to me

the main reason to get the drybox is to keep it from fungus..the cost servicing lens from fungus can go over the price of normal drybox..
still its up to u