I party disagree with the poster before. You can't really compare the SCJA exam with the SCJP - it makes perfect sense that SCJA is not a prerequisite for the SCJP because they are about completely different stuff.
SCJA will test you on superficial understanding of Java syntax and a broad knowledge of Java technologies. Just havea look at the topics covered. I feel like it' something like 'the small SCEA' exam. Code related questions are rare. maybe 5-10%. This exam is a sweet asset if you are trying to get a foot in the door and you are a junior member or your job is more planning related, like sotware project manager. It is easier than the SCJP in the way that you can learn more from the books and do not need to learn stuff the hard way as much as for the stuff for the SCJP
SCJP on the other side will test you intensively about syntax and programming code and even details of standard APIs. You absolutely have no chance clearing this exam without solid coding experience. The SCJP is a valuable certificate if you are going to work as a programmer. Well, since you earned this one, youll know that
However, I agree with Lucas that there are only few situations where a SCJA would be really useful. Which certification would be useful for you, totally depends of the area you work in. If you are really into all those architectural stuff and you have some years of experience, have a lok at the SCEA but else I'd go for another exam.
From a practical perspective: I can't really imagine many employers who actually are aware that the SCJA is not the small brother of SCJP ...