In general, the idea behind in-system programming in microcontrollers is to build the necessary programming support hardware into the final target device, not to encourage the construction of homebuilt device programmers. The hardware required for ISP, especially with this device, is intended to be truly minimal, so that its addition does not add significantly to the cost or size of the product it is used in. For most applications with serial communication support, adding in-system programming support can be achieved for literally the cost of a '125 tri-state buffer (well under a dollar). Sure, that's a few pennies not in your pocket for every product sold. But if you forego the ISP support in the target, every time you have to deploy a technician to a customer site for a firmware upgrade instead of having the user download firmware from your web site, a lot of dollars will be leaving your pocket.

--Sasha Jevtic