The re-sale value. For whatever reason you would like to sell your gpu, you have a market still interested in the gpu. After you mined if of course it will be worn down, but gamers can still use them and therefore still take advantage of them. But when a mining-only gpu worns down or its hashrate is not profitable anymore, no one will be interested in buying it second hand. It becomes useless and with no re-sale value. Also the guarantee of the mining-only gpus I've seen is very short, like 3 months or so.

If you don't use a gaming gpu for mining anymore you can still enjoy it's display benefits.