The 15" wouldn't have weighted that much more and lasted longer with OS X upgrades because of the dedicated graphics card.
For instance 10.8 will only run on certain older Mac's, those with good graphics.
If you want to make that 13" last longer, use a external USB keyboard and mouse, avoid the built in keyboard and trackpad as much as possible.
Also keep it covered and don't wear jewelry as it can scratch the case, body chemicals mar the surface.
Don't use the Superdrive as a DVD movie playback machine, use it sparingly.
Backup your data and don't move the computer while it's in operation as the hard drive is fragile.