The two reasons I've cooked up for why an audio amplifier is on the unit:
1. The device is meant to grow a whole house audio network. Your main A/V system can run the main room, but your Nexus Q devices in your bathroom, your den, and your kitchen ought not all need full AV setups as well.
2. One of the main use cases is to offer a digital welcoming mat at your site to anyone who comes by. I'm not sure what the system capabilities are for the Nexus Q, but plenty of tablets and mobile devices have pretty respectable displays: what they lack is speakers, because speakers by their nature are bulky and weighty devices. Nexus Q solves the most problematic part of nomadic computing, adequate sound: the only part of media consumption that is by it's nature required to be a part of the room infrastructure.
There's a freebie reason as well: it's cost is close enough to free. The audio chip, TAS5713, is listed as $2.15 in 1k units. They probably had to spend that again to upgrade the power supply, for which 25 of it's available 35 watts are dedicated to audio, and most of that cost gets sunk in larger passive components. The binding posts are probably nearly a dollar. There's a tiny bit more board real estate, a couple more passive components, but those are pennies. The overall cost is, I maintain, slight.
Adding more power would've been problematic: bigger power supply, perhaps bigger PCB traces, and the unvented passive-cooling-cum-metal-casing would have needed to be revamped to handle the significant heat-load. After 25 watts, audio goes from being a single chip IC to requiring discrete components, namely cooled and dedicated power switching transistors. 25 watts was as most as could be squeezed in easily.
You mention amplifier speakers in your post, ot, but M-Audio AV40's would be my goto compare, and at 20 watts/channel they're A) are not much of an upgrade and b) no slouch in their class, having more watts than many many AirPlay devices. Amplified speakers also create a much more complex wiring scenario: instead of being free to wire speakers wherever, one speaker now wants wall power, and the other speaker has to chain off the first. With an integrated amplifier, speakers are free to be moved into whatever satellite position is desired.
12.5 watts is quite a lot of power. It sounds meek compared to the hundred watts of an AV amplifier, and indeed, it won't get ragingly loud on most speaker sensitivities, but it will be loud (buy a Sonic Impact T-Amp and find out for yourself! $35 awesome-sauce battery-powerable mini amp, highly recommended!).
I for one think the ability to easily throw up cloud powered bookshelf speakers wherever is really cool.