SIEM market is comparable to messaging market in late 90s. Editors trying to impose their own format to the users and thus, impossibility to interoperate between SIEMs. This is becoming a real problem as it prevents interoperability between tools thus collaboration. Even more serious, the same SIEM tool implemented by different team will produce different security result also using the same taxonomy.
A standard is important for collaboration and interoperability but also for conformity.