When factoring your invoices obviously the cost is a concern. But it is a mistake to compare the 30 day discount rate with a normal annual percentage rate. In order to consider the normal APR a business has to qualify at a bank for an annualized loan. That usually means 3 years of tax returns showing profitability and a positive net-worth. If the business has large fluctuations in their sales and payments then the bank will only be able to loan a limited amount.
On the other hand factoring companies rely on the creditworthiness of your customers, so tax returns are unnecessary and a negative net-worth for an emerging company is not unheard of and welcomed as part of the growing process. So the cost of factoring is considered short term capital which is more expensive than the APR but much less expensive than equity investment.
As long as the profit margins are healthy enough, factoring is still a useful tool.