It is dangerous to have a power outlet run from a light circuit?
As ChrisF says, check your local codes on this one. The purpose of separate circuits is to ensure that you have lights when you plug in the vacuum cleaner, hair dryer, or some other high load device. If the lights don't have any outlets, and the circuit has been properly sized, the odds of them tripping and leaving you in the dark are slim.

When adding to an existing circuit, the important question is to be sure you're not overloading that circuit. You typically want high load devices on a dedicated circuit (e.g. sump pump, AC, refrigerator). For general purpose outlets, the rule of thumb is 8 outlets on a 15 amp circuit, or 10 on a 20 amp. With lighting, I think it's limited around 10-12 per circuit, but there's a big difference if you're putting low wattage fixtures (CFL/LED) vs high wattage incandescent.

The calculation for capacity of a breaker is a target max of 80% of the breaker capacity. So 15A breaker x 80% utilization = 12A target. 12A x 120V = 1440VA. The assumption is 180VA per receptacle, so 1440VA / 180VA = 8 receptacles.

Now, if you're adding a garage door opener to the circuit, and the label says 8A, then 8A x 120V = 960VA. 1440VA target on a 15A circuit - 960VA = 480VA remaining for other receptacles. 480VA remaining / 180VA per receptacle = 2.6 receptacles allowed if all you have is a 15 amp circuit, so you shouldn't branch off a circuit that already has more than two outlets. But if one of those receptacles is you running some power tools when someone opens the garage door, there's a good chance the breaker will trip anyway.