yup just dont go over half full total ?
I think many people like to keep a lean boot cause we feel it performs better and or other reasons ?
the more you fill up a HDD as I am sure yo know the faster it is ? so if you can keep a lean boot on a HDD and say only fill %10 the slow down will be minimal
OS X with journaling does a pretty good job of keeping things neat in the file system so that small 20-30 or so gigs for your file system stay way to the outside of your HDD
if you have everything on ONE larger as you use it ad more programs etc.. and lets say you use up %50 of the HDD now some of your programs etc.... are in the half way down part of the HDD meaning half way down % wise not physical but this may cause it to be a bit slower at times ? the other thing is pulling files off your boot as you are using programs is not as efficient in some cases ?
again theory and reality ? its hard to say for sure
so going with the partition you gain a for sure advantage (at least I think so and sure some others might agree)
also you need a backup so that extra space does not go to waste ?
check out this link
http://macperformanceguide.com/Storage-WhyYouNeedMoreThanYouNeed.html