OK, so maybe that’s a bit strong: you can live without them, but you certainly can’t travel and work reliably without them.
It is so important to have a backup internet connection, in many parts of the world wifi and even power can disappear without notice. Indeed, in many less developed parts of the world mobile connections are much more widespread and reliable than anything else; in Africa for example many places haven’t bothered with cabling (even for power) but everyone has a mobile phone.
Even in ‘developed’ countries you’ll find yourself grateful for the magic little thing at some point.
Here are some of my recent experiences:
I found the wifi connections in while travelling and working in Vietnam really good (much easier to find that in Europe, Japan or Australia – in fact, as easy as their visa on arrival to Vietnam). But it was still important to have a 3G backup connection; as it was less reliable in some of the places I stayed.
Mobifone and Viettel are two of the good service providers in Vietnam. I bought a Viettel dongle and sim card with a load of credit for about $30 US in Siagon. I recommend finding one of the many specialist phone places (in fact it heard it is simple to purchase at the airport, but I wasn’t that organised). Make sure you get them to activate it for you and then immediately test yourself.
Top up credit can be purchased almost everywhere; but I didn’t need to recharge until after two months.
The 3G speeds where pretty good across the country on the whole; plenty enough for most things including some Skyping.