Wide porcelain cups help to retain heat, especially with a cover of foam. It also provides a bigger surface area for latte art.
I prefer lattes in big wide cups for other reasons as well:
Two hands: before I drink, the process of consuming starts by absorbing the warmth into my hands. This tells me if the latte is too hot to consume. The process also conjures up memories of the Japanese green tea ceremony where two hands are used to drink. It makes me feel connected to traditions that are hundreds of years old.
Mouth feel: when my lips embrace the outer top of the cup, my tongue bridges against the lip of the cup. From there, I can take a small drink of the crema and foam. The mouth and tongue feel on smooth porcelain is a unique experience that neither a to-go cup nor a clear bar glass can provide.