I have a road, mountain and CX bike. I enjoy riding all of my bikes and ride them all pretty hard. When it's wet where I live I always ride my CX bike. It's faster on the road, fun on fire roads and SIGNIFICANTLY easier to clean than my MTB. Maintenance wise, you also don't have to deal with the added complexity of a shock. Lastly I feel you can get a quality CX bike cheaper than a MTB.
As others have already mentioned, if you're at all interested i going off road more often, MTBing is a blast. Riding down a dirt trail, fishtailing or taking jumps, it's a surefire way to feel like you're 12 again.