R’ Yaakov Emden very strongly wanted to do away with Cherem Rabbeinu Gershom altogether. But even if it is still binding, he held that פילגשים are permitted nowadays and he seems to think they are a good idea to have, in addition to a wife. He held that Cherem Rabbeinu Gershom does not forbid someone from taking a פילגש besides for his wife.
שאילת יעב”ץ ח”ב סי’ ט”ו
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