ummm AT&T uses odd ones as well. It is not T-Mobile US or AT&T fault for their "odd" 3G frequencies in the us.
AT&T was forced to use the one it users because Verizon bought up the frequencies that europe uses and T-Mobile got a double hit because AT&T owned that next block so they had to find another set they could use.
I would agree with you, perhaps I wasn't clear. I think it's just that the 1700MHz frequency really hurts things for the iPhone on T-Mobile, since in the grand scheme of things, it's not used much (only some small carriers in the Canada, and in the US only T-Mobile, and some areas for Cincinnati Bell).
Apple really wants to try to offer the broadest scope possible. I suspect once technology improves, adding the 1700MHz band would be easy for future models, so that way Apple wouldn't get into something like the early days where there were so many single or dual-band phones that were essentially the same GSM model, but a specific one for each carrier (take a look back at the Sony Ericsson T610/616/618 for a prime example).
Right now, here are the popular
frequencies broken down (GSM only):
800: Japan (NTT DoCoMo)
850: Americas (AT&T, Bell Mobility, Telcel, Telus, Rogers), Oceania (Telstra, Telecom NZ, Vodafone AU)
900: Europe, Asia, Oceania (Optus, Vodafone AU, Vodafone NZ), Dominican Republic (Orange), Venezuela (Digitel GSM)
1700: USA (T-Mobile, Cincinnati Bell Wireless), Canada (WIND Mobile, Mobilicity, Videotron), Chile (VTR, Nextel)
1800: Europe, Asia, Oceania (Falling out of use)
1900: Americas (AT&T, Bell Mobility, Telcel, Telus, Rogers)
2100: Europe, Asia, Africa, Oceania (Telstra, Optus, Vodafone AU & NZ, Three Mobile AU, 2° and Telecom NZ), Brazil
If you look at that list, 800, 850, 900, 1900, and 2100 covers most GSM carriers out there, which is what the iPhone 4 supports.
This will also get screwy when Verizon deploys their LTE network on the 700MHz spectrum...