Similar situation, different market. I applied to a BigLaw firm , Firm X, in law school, went through the interview process. Firm X was not the only firm not to extend an offer, but it was the only firm which never contacted me again after the in-office interview round.
Last fall, a foreign middle-market client (between $10m and $500m in assets) needed a US-based law firm for an m&a deal. Not a very large deal (relatively speaking) but it was an 8-figure deal that led to other 8-figure deals, with very good realization. The client was trying to choose between Firm X and several other firms; Firm X was their top choice simply because they'd heard of it. I told the client about my experiences with Firm X and speculated as to the quality of service they would receive as a middle-market client. The client dropped Firm X from their consideration list and went with another firm.
Not sending a free email or $0.35 letter cost Firm X several million in fees.