Because:
1) The version in there throws away valid blocks on p2pool
2) The version in there doesn't block the recent stratum redirect problem
3) The version in there passes all shares to the pool even if they are below target (I'm sure pools must hate the S2 due to the major increase in CPU requirements at the pool)
4) The version in there has the API set to W:0/0 so anyone with network access can change your settings/pool/username (and the web page doesn't let you fix that)
5) The version in there has a modified API with different field names to the standard API so anyone using other software that reads the API must get that software changed (or use a proper API version of cgminer in the S2 )
6) The version in there doesn't allow you to turn the beep off
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I released a new firmware yesterday with current cgminer (which has new S2 support), but it has severe problems on p2pool.
So once I've sorted that out, there will also be a solution