Good point. After all one cannot have both security and convenience. But careful when intutively using systems as throwing two dices. Often times the distributions are not as constant at it may seem.
Of course you can. I wouldn't say taking two minutes to generate a single high entropy seed using a deck of cards gives up either security or convenience.
Actually the seed is the beginning of a long algorithm you are trusting. And if you trust the algorithm, you can also trust the seed generator. If not, you need to generate for each private key 256 bits of entropy throwing a dice (for example).