I’d like to claim that leaving apostasy out of one’s understanding of the scriptures (be it systematic or informal) has several negative consequences. Here’s a few that come to mind (some of them are quite related):
1) It makes the OT harder to read. After all, apostasy plays a major role, and if one is not accustomed to thinking in terms of apostasy, many of the stories will seem strange.
2) It belittles the OT. Take point 1 a bit further, and one sees that the logical conclusion of seeing all these strange stories of apostasy while not thinking such a thing is possible in the NT is to belittle the importance of the OT to Christians today. After all, God was obviously angrier or something back then…
3) It turns rather straight-forward passages in the NT into problems that must be solved (often by twisting them until they have no real content, like saying they are offering threats that are only hypothetical and cannot actually happen).
4) For those in the reformed tradition, it makes them agnostic on issues of life and death. By leaving out apostasy, one is only left with the categories of elect and non-elect when discussing matters of life eternal. Though the covenant may play a role in one’s thinking, without apostasy it has little meaning in terms of a relevant category associated with salvation. Combined with the notion that only God knows who the elect are, denying apostasy denies the ability to know if one is saved (or if anyone else is saved, for that matter).
For a reformed treatment of apostasy, Joel’s treatment of the topic is a good place to start.