Like Faded and Mikuru said, it's a high risk move with very little reward. Any skilled player using a character with better DI than Zamus will come out of her throws unscathed about 80% of the time. That means that 1/5 grabs you get (against opponents with good air maneuverability), you'll actually be able to follow up with one or two attacks.
Shield grabs are particularly dangerous unless your opponent is in a lot of lag. She doesn't have many options out of her shield, which is a shame, but shield grabbing against people who don't use laggy moves will get you into trouble.
However, using it as a primary source of damage or a counter is not really what it was meant for. Instead, it should be used as a sort of mix-up when approaching on the ground. Dash attack to up-tilt will often catch people from spot dodges and some inexperienced players from their shields. Dash up-smash will usually catch spot dodgers. Dash grab will catch someone shielding. Instead of looking at this in terms of damage potential, you use it to make the opponent unsure of when you're going to throw that grab out, making it easier to score hits with the other 2 approaches.
Because the grab is so punishable, you should throw it out rarely (unless they're shielding all the time). Using it infrequently is probably when it is the most powerful in terms of mindgames, regardless of whether it is effective for damage. People will not expect it as much (though you should be EXTREMELY careful against spot dodge happy opponents and probably refrain from using it entirely), and it will be essentially a free 10% damage or so and stick the opponent in the air (though not in any advantageous position for you, it prevents them from being grounded temporarily, where most characters feel secure).
Keeping your opponent unable to guess your approach is important, and for that reason her grab is useful. It's definitely not effective for building damage in comparison to several other options, but it is better to do 10% damage to your opponent than it is to get punished by people that constantly drop their shield after you're in lag from one of your other dash approaches.