Yes, it is necessary. Some would even call it essential. The recommended RH for storing cameras is 45-55%. Higher humidity will be in favour of fungus, lower humidity might cause damage to rubber parts in the long run. So you need to keep an eye on that. No chance without RH meter.
yup, RH meters are necessary. thirsty hippos can't tell you how dry or moist the air in the box really is.
however, too dry an environment won't cause the rubber to crack per se, that's if you use your equipment often enough. i usually keep my RH between 35-45%. so far so good. no cracks. haha.
i would advise you to get a dry box if you're really serious about photography. as the years go by, the more gear you'll have. from DSLR bodies, to SLR bodies, to AF lenses, to MF lenses, to TLRs, to vintages cams, to toy cams, the list goes on and on. and you'll soon find yourself needing too much silica gel! but with that said, it's also better to get a bigger dry box right from the start. my dry box in my room is running out of space (but it's a hand me down from my mum, and since it's free, i can't really complain. haha)
so yes, get a dry box.
one last thing, silica gel is a cheaper alternative to thristy hippo and it's really good if you're going out for extended periods of time, ie to a very wet country or place. put the beads into a sock, tie it up (make sure the sock has no holes, or you'll be damn pissed afterwards) and chuck it with your gear. when you get back, heat the silica beads up, and the moisture will evaporate. silica gel is reuseable! hahaha.