it reduces resale value. Plus it is not like a ring that you will keep forever.
Heck no. While I don't really care about the resale price as I rarely resell my devices, the engraving detracts from the sleekness of the product. To me it's equivalent to those "wonderful" stickers you get on Windows machines, except you can't remove the engraving.
...or coming home with your brand new car only to announce to the world where you bought your car from and proceed to proudly display said stickers until you sell it.
My original iPad is engraved and I have no intention of selling/upgrading. Tried a new iPad and couldn't see enough difference to justify the expense of changing.
When it finally gives up the ghost, it will go in the skip after the use of a sledgehammer.
If anything, engraving something clever on the back of an iPad could actually increase resell value!