Changing lenses is the single best thing about having a DSLR, if you want 12x zoom and macro ability without ever changing lenses you want a bridge camera.
Changing lenses often isn't bad for the camera per se, I go through days when I do 10 or 20 lens changes. The wear on the mount is totally negligible. Whilst you obviously want to keep the inside of your camera clean, never changing lenses isn't the best way to go about it....
I'm afraid there is already a load of dust debris, tiny bits of plastic etc. inside the camera from when it was manufactured. The body is also not airtight even when a lens is mounted, and moving parts wear over time shedding tiny particulates that all contribute to dust.
The best way to keep it clean is to wipe the sensor with a special alcohol wipe when necessary. It's surprisingly easy (its worth noting that you're not wiping the sensor itself, but a hardened piece of glass that's stuck on the front. The need for this depends on what you're shooting - dust becomes more visible the more you stop down. I can go for months without cleaning if I'm mostly shooting in low light with wide apertures.
It's very hard for dust to irreversably ruin a photo (unlike a poor lens). It tends to only be visible against a plain background, which means it's easy to clone out in software. It's obviously better not to have the dust there, but the ability to get rid of it after a photo has been taken helps prevent me staying awake at night worrying about dust!