First of all there is the size issue. I have my Surface in a leather case similar to many of the iPad ones and it is great for reading. The weight difference with a SP3 is real. In my messenger bag, the S3 takes no space. Then there is the LTE and the brick. I wanted a device that took very little space for travel, court, etc. The SP4 when factor in the bricks and a hotspot really does take more. I also wanted to be able to charge it in coach on plane and still run into a fair amount of planes (e.g. Virgin coach) with only USB in coach. I also like the fact that I can have charging cables. For example, I'm using a 10 foot Anker braided USB cable which gives me a long cable without having carry the Surface Brick, plus an extension cord, plus the hotspot. Respectfully Turbo4AWD, it is not that clear.
This is not my main machine. This device is a companion device for me and I think the 3 fills that void better. What I would really like better is a Surface 4 with its current size, CoreM, USB-C charging, LTE, and an SSD drive. Again for me, it wasn't the price, but the form.
Stu
PS: Another funny thing I've run into is that one court that I practice in does not require prior judicial permission to bring or use a tablet in court, but requires special permission for "laptops." My Surface 3 slides through courthouse security perfectly and I have no problem using it in tablet format without special permission.