It's just hard, I only came across one which is the old Mac Book, but newer models seems to have it removed. Nowadays, even 4-pin port is not very common, and I guess maybe it's time to get one before it obsolete?
Yesterday, I tried to find some longer 6-pin to 6-pin 1394 cables and it's also hard. Shorter cables are everywhere basically, but longer cables were only sold from one or two shops. Unluckily, every time when I ask, I was told that it was all sold. I couldn't buy any as well.
Without a long 6-pin to 6-pin cable, a 1394 hubs is therefore a must, just to extend the cable length, but then also means adding power through the hubs is not much trouble as well, since I will need that hubs anyway. Therefore, it makes a 4-pin notebook equally attractive.