The default value of 4096 has been identified as the optimum blocksize for the index. In fact, this was the only option for the longest time, and an option to use 8192 was introduced for a specific, unique situation. The tradeoff here is that while you might save very minimally on your I/O to the index, you will also increase your CPU usage and search times scanning the index records.
Consequently, there is no real benefit in increasing the blocksize for most databases, and there is a possible detriment to overall performance. In addition, unless the blocksize for every database is changed to this higher value, there is a potential increase in the amount of storage used for index buffers, and a possibility that half of this buffer storage is wasted by having to allocate buffers at the largest blocksize.