>Explain to me why ext3 is compiled as a module in the debian kernels
A better question would be why it's there at all, given that the ext4 driver is supposed to handle ext3 just as well (unless my understanding is wrong; that's quite possible). But it does make some sense - eg if you're using btrfs / (which I hear is all the rage these days, don'tcherknow) you likely have no need to read ext3 filesystems.
And having just looked at the result of lsmod to find out what ext? modules are loaded, I'm now wondering why I seem to have [at least] the following fs modules *all* loaded:
fuse, btrfs, ufs, qnx4, hfsplus, hfs, minix, ntfs, vfat, msdos, fat, jfs, xfs, exportfs, reiserfs, ext3, ext2, isofs, udf, ext4
Not to mention a bajillion other modules for hardware I don't have.