Your wallet doesn't technically contain your bitcoins.  It only contains the keys which authorize you to use them.  There is one key per address, so you should only need to create a backup whenever you create a new address.  (I've been told that when you first create a wallet, multiple addresses are pre-made under the hood, which means that after a single backup, you're good until you've made more addresses beyond this initial amount.)

And yes, you can definitely reuse the same address as much as you want.  Addresses are basically like public boxes that anyone can see inside, learn the history of, and deposit bitcoins into, but only the person with the key can pull bitcoins out of.  Reusing a single address lets everyone else also easily determine that all the transactions into and out of that box are related by all being to or from the same person, thus decreasing anonymity.  And if all your money is stored behind a single address, then if someone else acquires your key to just that one address, then they have complete access to your money.  Keeping money in multiple addresses helps avoid this, except that a single wallet can contain many keys, so that to really be secure from key theft, it helps to have multiple wallets, each with just a few addresses in them.  But that complicates things a lot, obviously.  It's a bit of a tradeoff between increasing security and increasing convenience.  (They're both pretty high as it is, but if you want one to be even higher, then the other typically has to go down some.)

Disclaimer:  I am a bitcoin noob.  I cannot guarantee that I am completely accurate.