I thought RAID0 gives better performance (plus a larger disk area), why
isn't it a good option?
Because that results in all mass storage being lost with
either drive (if there had been two) failing. Any
well-engineered data storage should have at least a minimal
level of redundancy. IF you are aggressively backing up
that data to other drives, that might be sufficient- I can't
make that call but always err towards a conservative stance
when it comes to other people's data.
RAID does not give more disk area, you always end up with
the same amount or less depending on RAID level. If you
meant a single larger partition, most advanced users find
that the opposite of desireable.
Further, with two physical drives it's usually faster to
allocate usage of the two such that while performing a job,
both drives are in use for different purposes. For example,
one drive has OS, the other apps, and whichever is least
burdened, the application data. Within more demanding apps,
the source and destinations for larger files can be split
between the two physical drives too. This does not apply to
mutiple partitions on same drive nearly as much as different
drives.
Add to that the real possiblity of drive failure due to the
more harsh environment due to the mobility of laptops and
the risk escalates.