Here’s my advice:
If you have any rare DVD burners that can’t be easily replaced, like a BenQ DW1640, or a special drive that burns so well it seems anointed by Jesus Christ, like a Pioneer DVR-115D, only use those for burning, and don’t add to the MTBF by using them as readers. Instead, grab a cheap ghetto drive off eBay or in the clearance bin somewhere, and use that for your reading.
On the other hand, if you have an older drive that is mediocre, like almost anything made by ASUS, BTC, or Optiarc (pre-7200x), or a newer drive that is super cheap and easy to replace, like a Lite-On 20A4P, it doesn’t matter if you use them for both reading and writing. If they break, just buy another or upgrade to something better.
For example, if you were currently using an Optiarc AD-7190, you could feel free to abuse and treat the drive poorly. Go ahead and bang it up, throw it around, and treat it like crap. Use it as a reader, writer, or drug dealer, it wouldn’t matter. If the drive broke, you’d just go right ahead and get a super affordable and shiny new Optiarc AD-7200, which is a superior drive. Now, once you got the AD-7200, you’d have to be gentle with it, and treat it like royalty, only using it for burning and nothing else. Understand?