a desktop drive's life expectancy is usually defined through the number of times the head is unparked/parked in the so called "landing zone". so one could argue that less usage ultimately leads to longer life. but there are several other components than the platter and the head that can fail: motor, bearing, controller chip, cache memory....
at work i often see perfectly working old drives (4y+) not spinning up again after a server hot or cold restart. usually a problem with the motor or bearing being worn off too much.
no raid level will prevent a drive from failing, but it will make a data loss less likely to occur. the important thing is that you backup your data regularly. raid levels >=1 are no replacement for backup!
imho you should consider RAID 5 as you will only loose 1/n of your disk capacity, n standing for the number of drives in the raid 5 array > 2. raid 1 is cool, too. but i do not use that very often. maybe on systems where the parity calculations (xor-operation) can cause problems (such as database servers) or operating system disks.
out of my point of view, harddrives are expendable items. it only gets messy should you expierence any kind of data loss.