The quality implication of the current line:

The average waiting time is $47$ minutes which is very high. There are, on an average, $2\mathrm{.}94$ people waiting always. This is an awful situation from the point of view of customer satisfaction.

Would you consider the seventh server:

From the quality point of view, definitely yes. The average waiting time reduces to $12\mathrm{.}97$ minutes and only $0\mathrm{.}81$ people, on average, are waiting. However, the benefit in terms of quality and goodwill must be weighed against the additional cost of the seventh server.

Is utilization a factor?

Utilization is definitely a factor in that the payout to the servers is not based on effective working time but based on the overall reporting time. So$,$ the lesser the utilization, the more the loss from the owner's point of view because of the cost of undertime increases due to less utilization.

Is cost a factor?

Cost is always a factor. How much cost is associated with the customers' waiting time depends on the business level strategy of the restaurant. For example, if the restaurant is too eager to maintain a high quality and high customer service, then it will assign a high waiting time cost and then the seventh server may be feasible $($the cost of the server will be outweighed by the reduction in waiting time cost$).$