Sign language is just as much of a language as any spoken language. People with aphasia who know how to sign are affected just as much as those who speak spoken languages. Also cognitive stress affects signers who are not native speakers. For example, a pilot who was not a native speaker of English would have a hard time speaking to his passengers in English if there is an issue with the plane. Finally and most importantly, deaf people who do not learn to sign, and are forced to read lips almost never go past an elementary level in their reading a writing abilities. Just as somebody who was never fully immersed in a language.
When two people who know ASL sign to each other, their spatial relation to each other is important. Signers "speak" from their location, therefore the listener needs to make a 180 degree spatial rotation. Due to the fact that signers have to do this every day, they excel at spatial reasoning. From this article at Live Science:
Knowledge of American Sign Language comes in handy when studying structural geology.However later in the article this is mentioned:
Come again? Structural geologists have to visualize the bending, breaking and folding of rock formations that are usually motionless and firm. This often requires the processing of complex spatial information—something that individuals experienced in American Sign Language, or ASL, already do well, explained Michele Cooke, a geologist at the University of Massachusetts Amherst.
The reason why these results are inconclusive is due to no spatial reasoning being used by the students with their gestures. Simply gesturing with your hands will not help. One would have to use the some cues in ASL or actually start using sign language.
Wondering if the relationship between signing and spatial reasoning can be further validated, Cooke now tries to incorporate "ASL-based gestures" into the classroom by encouraging her students to communicate with their hands during discussions.
"So far our results have been inconclusive; the students are resistant to this kind of learning," Cooke said. "They need to get rid of their inhibitions and not feel intimidated by using their hands when they participate in class."
TL;DR - People who know sign language kick ass at Tetris.