I used to use my 17" 2.33Ghz MBP as a makeshift broadcast monitor on video shoots, and having 2 batteries in those instances proved useful on exactly 1-2 occasions.. purely because i had forgotten to charge the main one. Most of the time it just meant one less power outlet to look for, i can't think of many people who truly need to be able to change batteries especially if the battery lasted 5 hours.
I find the Air a great performer when it comes to battery power, and only on the occasional longhaul flight i take would i ever want the need to extend the battery life, but honestly i think the majority of people who do frequent longhaul flights and have macbook airs would be flying business class anyway and battery life will mean the difference between carrying a power brick and a spare battery. If the new 17" has the same performance/battery life ratio as the Air, i would personally be very happy with it, and if it means making it even thinner, then for that reason alone i think its worth it, laptops are meant to portable, not just desktop replacements.
But if the battery life on the 17" is increased by only 50% by being built in, i would personally see that too as reason enough to make it built in, and for the rare person that needs to use their 17" laptop in an environment away from any form of power supply for more than 5-6 hours, then they could always carry something like brianbobcat suggested, an external battery that connected through the magsafe connector. I think the majority of users will prefer having a product that was thinner, lighter and lasted 50% longer rather than having to carry another battery.. (btw am i the only one to cut themselves with the current MBP battery? the corners are like razors!!)
The ONLY real disadvantage i see is when people need to get the battery replaced, i can't imagine there being too many people who's lives will be massively affected by sending their ipod away for 2-3 days to get the battery replaced, but i could imagine it being a fairly big inconvenience for someone to send their laptop away.. but then again those that HAVE to use their laptop away from a power source are usually the kind of people to have more than one laptop/computer anyway, so even in that situation i can't imagine it being too big a hassle.. the iphone, that's a different story.
And i've heard many horror stories about faulty batteries, notably on the previous 15" MBPs, but i haven't heard them for the Air, ipods and iphone, so hopefully that wouldn't be the same for a built in 17" battery.
Thumbs up for Built-in Battery!! i can think of some shortcomings in Apple's software choices (iPhone COPY AND PASTE!!!!), but i can rarely fault their Hardware design, most passionate fanboys i know complain at rumors but when they start to actually use the product, they tend to shut up and start to realise the genius of the design choices.. hence why they (including myself) became fanboys in the first place. Apple design may sometimes eschew practicality, but if it didn't, then there would be nothing unique about Apple's products, Ferrari's are great performers but try go through a mcdonald's drive thru with one, and you'll wish you had an SUV, a car that got you and your kids from A-B, but with far less style and panache... for the people that MUST have removable batteries, i doubt it will be much trouble to get a Dell laptop and Hackintosh it, but for the rest i think this is a good decision, and i doubt Apple's shareholders are going to complain about a design element that causes more foot traffic in the retail stores.
All these complaints, and this thing will still sell well..... If you don't like it, then don't buy it. That's the only thing Apple will respond to.