The question is not whether an attack may be good or bad for the pool manager (I have the same opinion as pekatete on that) but whether this with-hold attack has really taken place!
Phew, thats a relief! Somebody actually has an opinion congruent to mine on at least one aspect.
If slush had increased the power displayed his pool for exemple +10PH (no one can know if the indication is true), it dims the incomes of miners and pool keeps the difference.
I fundamentally dis-agree with this, both in terms of the motive for slush to "lie" about the hashing power of the pool, and in terms of dimming / reducing the incomes of miners. For example, if a pool displays its hash power as 30PH when it is actually 20PH
1. The pool will only be earning circa 20PH worth of rewards
2. The pool will report lower luck because it is only earning 20PH worth, which is less than what is computed for 30PH
3. The miners will only be paid what the pool has earned
4. Miners will STILL be earning proportionately to their real hash power and therefore will NOT loose anything.
Unless you are saying that slush's setup is by far very superior to any other pool / mining operation on the network to an extent that he needs to fake his hash power so that pool luck reported is within the generally published values (but on this accassion something went wrong), then your assertion is flawed. I can not fathom any other plausible reason why slush would want to inflate the pool hash power.