1) Focus. Your business is selling stuff, not installing and managing a complicated script. You'll have enough issues running a business that you won't be able to outsource to someone else the way you can with a shopping cart script.
2) Price. Paid hosted shopping cart software's price is inconsequential: $10-$100/month. If that's a significant expense for your business, you need to look into improving your profit margin.
3) Support. While support for "paid" business software is included in the cheap price, support for "free" business software is pretty expensive. OpenSource and other free software come without any support except forums for do-it-yourself-ers to share ideas. So, you'll end up paying someone to install and maintain it. Or you do it yourself, which costs so much time it's more expensive than paying someone if your time is worth anything.
4) Installation. When you buy hosted software on a subscription plan, installation is already done and support is included. When you have an issue–and have you ever had software that you didn't have an issue with at least once?–you don't have to pay a developer $150/hour to make it go away.
5) Security. Security issues are taken care of for you by the provider–no patching software.
6) Speed. Hosted online shopping carts really do work now, out of the box. You just input your inventory and go. OpenSource shopping carts come with dozens of customization options that will take you hours just to decide you don't need most of them.
7) Peace of mind. The shopping cart is the jugular vein of an online business. If anything makes the blood stop flowing, however minor the cause, you'll feel it fast. If you are the one who has to make sure the blood stays flowing, or if you have to rely on the availability of a freelance programmer, you won't be getting the soundest sleep. Let a specialist do that worrying for you, 24 hours a day.
