The biggest unknown right now is how this will affect the standby AND regular Fastpass users. Sure, you don't HAVE to use xPASS. But until we know how it affects those not using it, not using it could be a distinct disadvantage - much like with ADRs (at least in the past).
If xPASS slots are in addition to existing Fastpass slots, then there will be fewer standby slots, and standby loses. Fastpass also loses somewhat because there will be more people entering the Fastpass/xPASS line.
If xPASS replaces some portion of the Fastpass slots, then standby is OK, but Fastpass users lose out as there will be less available.
If xPASS replaces Fastpass completely, then non-xPASS users have no choice but to stand in line.
We don't know which of the above scenarios is taking place. But unless I'm missing a scenario, someone has to lose. At best they could limit the impact if there are limits on how xPASS can be used - for instance, if you are using xPASS, you CANNOT also get a Fastpass, etc.