You don't always see through portals. Sometimes they seem to be bugged for reasons I do not know. But more commonly, it's because the portals are too far away. When they're close, you can almost always see through them. I assume not seeing through them when they're far away is to prevent slow downs, or it may just be because the chunk unloads.
Also the see through portals setting may be disabled in Mindcrack. I'm pretty sure it's not disabled by default, since AnderZEL (Mindcrack Server member) got a Portal Gun and I'm fairly sure you could see through his portals in his videos. I don't remember for certain though.
