Basically, when you wipe data/cache/dalvik, your current ROM still exists, and you can even boot up. What you essentially did was a factory reset for your current ROM. If you have a custom ROM, you'll still have it, but with stock apps/settings. The reason is that the ROM exists in /system. /boot contains information necessary for the ROM to boot up (kernel, etc.). When you flash a ROM, it's overwriting what is in /boot and /system, and generally not touching /data. You have the wipe /data because there can be major conflicts with what's in /data with a new ROM. So, when you wipe data, you're not wiping out the old ROM; you're just hoping to eliminate data/settings conflicts. In order to actually wipe the old ROM before flashing a new one, you must wipe /system and /boot. Up until RA 2.3, you need to flash calkulin's Formal All zip, which would wipe /system and /boot. Is this necessary? Well, maybe. It's recommended, especially when switching between Sense and AOSP, and vice versa. Do most people do it? No, not really. If you have a problem with a new ROM, consider wiping /system and /boot. Otherwise, you should be fine not doing it.
