Why do startups need to implement a complex billing solution?

Assume that the initial implementation of an in-house SaaS billing UI takes about 8 weeks compared to a week for a hosted or embedded solution. Basically, It is time which startups could use to improve their product. To create a complex pricing: trials, coupons, discounts; subscription management with well-designed UI; dunning; analytics and so on, would you prefere a ready-to-use decision?