You are on this list and don't want your money trapped in limbo.
https://blockchain.info/unconfirmed-transactionsYou've made a copypasta error and sent money to the wrong address, yet it has not been confirmed yet.
Someone has hacked your computer and sent your money to an address that you do not control, yet it has not been confirmed yet.
Yes, I came up with all these reasons myself, but they just don't hold up to scrutiny in my mind.
1. I've had money trapped in limbo, and it was annoying, but it eventually got confirmed. If it didn't then the other nodes would have dropped it from their memory pool. Neither I nor the recipient was ever at risk of losing funds. When we have smarter miners that implement the "child pays parent" scheme then this annoyance should disappear.
2 & 3. Both these arguments seem like slippery-slopes: what if it has only received 1 confirmation? Is it ethical to pay a miner to attempt to orphan this block? From an ethical perspective, I believe that the transaction becomes legitimate the moment it is broadcast to the network. Again, just because it is technically possible to intentionally orphan a block or swap an earlier TX variant with a later one, doesn't mean that doing so is ethical.
Additional thoughts:
Copypasta: I hope that in most cases, copypasta errors don't result in the loss of funds. I only store addresses in my address-book of people I trust: if I made a copypasta error, I would simply ask for my funds back. (Readers should note that typo errors are safe, as the typo will result in an invalid bitcoin address and your wallet will flag the error.) Furthermore, as wallet technology improves, this will naturally become less of a risk.
Theft: If hackers stole the limited funds accessible on my computer...hmm that reminds me that I need to make another transaction to cold storage. I don't know. This is the hardest one for me, and...you know...I would probably try to double-spend in this case if I had the ability to attempt it. But I would rather not have this ability if it means that others do not have this ability. For this reason, I don't believe I would ever accept the later TX variant into my memory pool. That being said, if a hacker breaks into my computer and steals my bitcoin, they still committed a crime!