One of the most appealing aspects of owning a tablet from ASUS' Transformer line is the optional keyboard dock. It turns the tablet into an Android-powered netbook of sorts, making note taking, replying to emails, and other general productivity-based things much easier. The included battery also boosts usable time by up to four+ hours on most models, making the dock even more useful.
The main problem with the keyboard accessory, however, is the price tag; after spending $400+ on a new tablet, many users don't want to shell out the extra $150 to add the dock. After all, for $550, you can get a [somewhat] decent low-end laptop.