On the one hand multiband support is nice if you buy local SIMs and so affordable data tariffs and on the other hand there are networks like Telstra's in Australia, which have a far better 3G-coverage than 2G. Just compare these two coverage maps:
UMTS850:
GSM Coverage Maps | Australia | Telstra Corporation Limited (Telstra MobileNet) | Coverage Map (it's UMTS850 only, despite the map description says "3G 850 + GSM")
GSM900/1800:
GSM Coverage Maps | Australia | Telstra Corporation Limited (Telstra MobileNet) | Coverage Map
If you further compare Telstra's UMTS850 coverage to the GSM coverage of Telstra's competitors...
GSM Coverage Maps | Australia | Vodafone Pacific Limited | Coverage Map
GSM Coverage Maps | Australia | Singtel Optus Limited (YES OPTUS) | Coverage Map
...you see that with a UMTS850-phone you'll get the best coverage in Australia by far.
Another advantage of UMTS is, that those networks have higher capacity, which has often allowed me to place and receive calls, when 2G networks were congested (especially during holidays or events).