Cant see why it would put anymore strain on your laser from playing backups unless your using really tack media (which even then shouldnt cause a problem) Ive been playin backups on my dvd player for around 18 months now day in day out not had no problems at all, also with the price of dvd players its really not worth worrying about you can pick them up for less then what you would pay for 2 retail dvd's Cheers!!!
Techincally playing burned discs is a little bit more work for your player because there are generally more errors on them and the surface is not as reflective to begin with (of course if you play a disc from the store that's been scratched it's also more work and people do that all the time). It may decrease the life of the laser, but not by enough that you should notice unless it's a piece of crap to begin with. Honestly, on many players the mechanical parts are likely to wear out before the laser anyway so I don't consider it worth worrying about unless you're trying to baby a player that's already dying.