The longer the contract term, the greater the likelihood that some variation to the contract will need to be negotiated. Change is almost inevitably more expensive when it takes place by way of a variation to an existing contract, than if it is done by way of competitive tendering. This is particularly the case if the nature of the change was not envisaged at the time the contract was drawn up.11
Under conventional procurement, service contracts can be let for shorter periods of time, eg, 5 years. Any change in requirements by the client can be incorporated into the contract at the time of the next re-tendering and priced competitively.
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11 http://www.hm-treasury.gov.uk/d/pfi_change_protocol_principles.pdf sets out some principles that are helpful in developing appropriate contractual provisions for contract variations that can be anticipated.