I had a student ask this very question recently in class: won't more Cubans start coming to the United States, knowing that immigration policy is likely to change? The answer is yes.
The migration route is not new for Cubans. But the numbers passing through over the past month have grown to the point that human rights activists in Mexico have labeled it a “migration crisis” that is adding to the already high number of Central American migrants also using Mexican land as a pathway toward America.
It's all about incentives. If you provide a privileged place for Cubans and give the sense that you will remove it, then of course people will feel like now is their chance because the option will disappear.
As I've written before, Cubans should not receive preferential treatment. Meanwhile, even ardent anti-Castroites acknowledge that the Cuban Adjustment Act is widely abused. The best solution would be to make the decision now before more people start making expensive and dangerous treks, often with their children.
Update: Ileana Ros-Lehtinen also just talked about problems with the law.