The 17" is ideal for an integrated battery, as it's a desktop replacement. The 15" however, fits a different target market. It's one of the most portable 15" notebooks in the world, and as such people want easy access to the battery to extend their productivity.
The MacBook Air is even more portable, and it doesn't have a removable battery.
Look at the CPU and clock speeds. Apple is spending less now than ever on the CPUs. There are much nicer CPUs available for the Mac notebooks, yet Apple has stuck with Core 2 Duo and around 2.4 GHz for the MBP for THREE YEARS! The MB has has a slower CPU than it did 18 months ago. It was 2.1 and 2.4 GHz, now is 2.0 and 2.4 GHz, and they cost more to get the 2.4 GHz.
The current $1299 MacBook has the same GHz CPU as the first $1299 MacBook.
More details for anyone who is interested (from an earlier post):
2006 Q2: At the original MacBook release, the line used the top 3 CPUs.
(MBP: $400/$600, MB: $300/$400)
2006 Q4: The line used the top 4 CPUs.
(MBP: $400/$600, MB: $240/$300)
2007 Q2: MBPs still used the top 2 CPUs, and the MBs went back to $300/$400 CPUs, although those would soon be outdated.
(MBP: $300/$500, MB: $300/$400)
2007 Q4: Intel bumped the clocks by 200 MHz in Q3, but Apple kept the same clocks while putting the 2.6 GHz as a BTO in the MBP. The MB gets current gen CPUs.
(MBP: $240/$300, MB: $240)
2008 Q1: Rather than going back to the top 2 CPUs for the MBP, Apple keeps the Q4 price ranges.
(MBP: $240/$300, MB: $200/$240)
2008 Q3: MBP and high-end MB CPU price ranges stay similar, while the $1299 MB uses an OEM CPU that's not even listed (I'm assuming sub-$200), however this could be excused due to the GPU. Same goes for the $999 one when it is updated to the NVIDIA chipset.
(MBP: $240/$340, MB: ≤$200/$240)
2009 Q1: Intel bumped the clocks by 133 MHz, but strangely only the top two MacBook Pros received the upgrade.
(MBP: $200/$340, MB: ≤$200/$200)
Recent MacBook Pros are pretty much using similar-range (and similar-priced) CPUs as early MacBooks.
If Apple uses $300~$500 CPUs now, we would see 2.67/2.93 GHz in the MBP and 2.53/2.67 GHz in the MB. If Apple had used ≤$200~$340 CPUs in the original MacBook series, we would have seen 1.67/1.83 GHz in the MBP (incidentally its original clock speeds) and 1.66 GHz single-core/dual-core in the MB.
How about the iMac. Apple could have a full line of Nehalem desktop computers right now if it used the desktop Core i7 CPU in the iMacs. Why in the world would Apple do this, to save money.
Core i7 is much too hot for the iMac, but there are several other ways that are still better than the 133 MHz speed bump for half the line.
a) Improved the cooling and used the 65 W desktop quads instead
b) Used the existing mobile quads, even as a BTO
c) Asked Intel for a custom faster CPU like they did the last update
d) None of the above, because they don't seem to care that much about higher speeds or quad-core for the iMac
You have your answer folks.