Biking in general is much more versatile. Here are some condition that a bike will handle just fine (with proper equipment) and the skates will not:

- very rough pavement/no pavement
- very steep downhills
- rain/snow/wet ground/ice
- heavy traffic

I think skates are really fun and would love to be able to skate more for transportation, but I find that in a lot of cases it's simply not practicable. Some of it is due to my lack of skills to tackle some of the more challenging conditions, but a lot of it simply can't be done on skates. If you live in a place where it never snows and hardly ever rains, the pavement/sidewalk is perfect everywhere and the hills aren't too steep, you could, probably, get away with skates as your main wheels, although I personally would still prefer at least a cheap basic bike as a back-up for when skating for whatever reason is not feasible.