Traditional, or conventional, non-serviced offices have various advantages, for the right sized company. Though the initial outlay can be more, running costs can be significantly reduced.
Some types of company do not need much in the way of customer-interface or the additional services that a serviced office can offer. In such cases, a conventional (non-serviced) office is probably more cost effective, provided favorable lease arrangements can be found.
More generally, traditional office space is likely to be more suitable when your company reaches a certain size (say 20 people) and you are fairly certain of your likely growth over the next two to three years. Although there will be substantial initial capital costs in taking a traditional lease, your monthly outgoings are likely to be lower.