By Ross Kerber Feb 16 (Reuters) - Compensation for S&P 500 .SPX chief executives has soared in recent years even as investors cast more of their advisory "Say on Pay" votes against management, leading to doubts about the ballots' usefulness. The conclusion amounts to a warning for corporate directors who will face investor judgements as the springtime annual shareholder meeting season gets under way, amid continued inflation and worker layoffs. Discovery Inc had multiple share classes that boosted insiders' power and voted on Zaslav's pay only once every three years, effectively insulating him from shareholder pressure, Weaver said.