For serious work, I think some kind of RAID + a backup strategy is necessary. You don't need a ton of space for it, just enough RAID volume to keep whatever current projects you have safe, and a place to make periodic backups to another drive, or preferably another machine.
You might not have the RAID option on a laptop with only a single drive -- I happen to have a smallish partition on an MSATA SSD on my laptop, but if you didn't have that option it would even be worthwhile to get creative. Maybe set up a periodic copy of your important directories to an SD card; its not RAID, but it's better than nothing in a pinch. If you had a couple USB 3.0 slots and a couple low-profile memory sticks you could even do a raid volume of those and leave them in place. At the very least you'd want to plug into your network (or cloud storage, if that's your bag) and sync your files every day or two if you absolutely have to fly without raid.
RAID 1 (mirroring, two+ drives) is probably sufficient for most purposes. RAID 5 (parity, three+ drives) is more space/cost-effective, but no more secure to drive failures (in either case, you have to lose two drives to be SOL in a minimal configuration), but does provide an additional measure of error detection and correction. Personally, I think you need to get into larger arrays of RAID 5 (like say, 3 data + 2 parity drives) before it wins out, and at that point you're spending some serious cash and would probably be looking at RAID 6 anyhow (distributed parity, 3+ drives). You can mirror (RAID 1) three drives if you wanted, and that would give you a degree of error-detection/correction too.
