  The functionality that distributed ledger technology provides, i.e., an
immutable and fraud-resistant registry with validation and verification
mechanisms, has traditionally been implemented with a trusted third party. Due
to the distributed nature of ledger technology, there is a strong recent trend
towards using ledgers to implement novel decentralized applications for a wide
range of use cases, e.g., in the financial sector and sharing economy. While
there can be several arguments for the use of a ledger, the key question is
whether it can fully replace any single trusted party in the system as
otherwise a (potentially simpler) solution can be built around the trusted
party. In this paper, we introduce an abstract view on ledger use cases and
present two fundamental criteria that must be met for any use case to be
implemented using a ledger-based approach without having to rely on any
particular party in the system. Moreover, we evaluate several ledger use cases
that have recently received considerable attention according to these criteria,
revealing that often participants need to trust each other despite using a
distributed ledger. Consequently, the potential of using a ledger as a
replacement for a trusted party is limited for these use cases.
