WiFi works great for me at home and work since I control those networks. But finding & connecting to public hotspots is often a pain and my effort's frequently rewarded with a slow, unstable connection (even for paid ones). In my area, LTE isn't marginally faster, it's way faster than 3G. For me, it's absolutely worth the extra money.

I really don't think having fast, hassle-free internet access on the go is an edge case. Isn't that one of the main reasons people buy smartphones?