Ok first off, we all don't have Verizon, and until Verizon's rather recent acquisition of Alltel, Verizon's network wasn't that great.
Now, Next.
If I pay for an unlimited Data plan, I'm going to use it. ATT's network is what it is because they are slow in the upgrading department, Not because of people watching You Tube. ATT has always been slower than everyone else when it comes to bandwidth, even on landlines. The ATT backbones servers are weak and antiquated. THAT is why ATT is slow, not it's users. And if you really need me to explain why thier backbone is so important, that's for another post.
Now crowding a certain cell tower is very possible and happens more often then you think. That is often the reason for dropped calls in good coverage areas in large cities. Even medium size cities. Thats why ATT/Tmobile phones will be harder to use in large metro events than Sprint/Verizon phones. Why? GSM Towers cant hold as many calls as CDMA towers can, and have less broadcast range. That's also why CDMA coverage is better than GSM. That is also why ATT/Tmobile will be switching to future CDMA technologies in the future. They already started with UMTS, and yes, that is a CDMA tech.
Now for the OP: Wifi can come in handy many many ways. On my WinMo phones, ( I don't suspect long before someone develops a similar application for the Android platform ) I could actually turn my phone into a WiFi hot spot to use with my laptop! Wireless tethering. Greatness. Also with WiFi, say your in a spot with shotty or bad coverage, you can use a VOIP client such as skype, or use other messenger services to communicate.