Although 'full-time' board membership provides a greater level of detail and understanding to the incumbent, it also detracts from the role of the board member, i.e. ensuring compliance, probity, legality, risk management, and adherence to and delivery of set goals.

When you're very close to the action, you inevitably become part of the action and as a board member, you then have to adjudicate over your own decisions, actions and performance.
Extensive research has demonstrated that managers and directors act subjectively (but with well-meaning intent.) Such subjectivity needs to be effectively over-sighted in the interests of the company and its shareholders.

Further, different companies in different contexts at different stages of their development need different inputs from their directors. There is no universal truth here.