Why Link2SD requires second partition on sdcard for linking? Why can't it simply link to extSdCard?
If it would use the only partition of your SDCard, apps would become unavailable if that card is mounted to e.g. your PC via UMS (USB mass storage). That's what happens with App2SD, and Link2SD wants to avoid that.
Ever tried creating links on FAT (which is the file-system used on your card). This adds to the problem.
Using a separate partition, Link2SD can make use of "advanced" file-system types, e.g. ext3/ext4, which leads (amongst other things) to better performance. Doing so with your only SDCard partition, this would introduce problems for Windows users, as Windows doesn't support EXT file systems out-of-the-box


These are the most obvious reasons for this decision. It might very well be I missed some more -- but these alone are enough to make it a wise decision :)