If a filter is enabled, then at least one of the "Actions to take when matched" boxes should be checked. This should be validated when a filter is being created/edited.
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Description
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I'm not sure this change would be very well received by the community. Sometimes, for various reasions, people only want to log actions. Recently, this was done on enwiki in https://en.wikipedia.org/wiki/Special:AbuseFilter/905 for T32750/T188960.
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Personally, I don't agree with this. It may happen, as pointed out by Nirmos, that you want a log only filter, especially if you want to test if it works well.
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You are absolutely correct. I had completely ignored the "silent alert" mode, which I myself have used in the past too.