If I understood correctly (bear in mind that this rarely happens

) fluffypony was referring to the possibility of MITM attacks.
In part yes, but also malware on your computer. There is no substitute for cold storage, paper wallets, good hardware wallets, etc.
So, afaik, there is nothing less secure with the wallets created with mymonero (discounting the 13 vs 24 word mnemonic).
If you, for whatever reason, used mymonero to create a coldish wallet, and never login again, you should be pretty safeish.
That is true assuming you are completely sure that nothing was compromised when you created it. Hard to really know that.
Well, what I wanted to convey is that, if one has already created the wallet for cold storage in mymonero, either the coins are already gone or they are still there.
Therefore, accessing mymonero just to transfer the coins to another cold storage may in fact worsen your chances of getting hacked. It would be safer to wait for the code to restore short mnemonic seeds to be available for offline use. Either through simplewallet or a third party client.