I'm not sure compiler writers care that much; they don't have to support every x86 instruction and can restrict themselves to whatever subset produces fast code.

Where it really kills you are things that have to analyze x86 binary code (e.g., binary static or dynamic analysis tools). Unless you cover every nook and cranny of x86, chances are you'll eventually encounter some program that uses one of these instructions and your analysis will break. And of course emulators like QEMU face the same problem.

This problem becomes especially severe in the context of malware – any instruction you forget to model becomes a way for a malicious program to evade your analysis.