I didn't mention true half-sword, that's a different thing. Though, that can also, in theory, be done with a true great sword, it is more the purview of the slightly smaller longswords.
True great swords are a very interesting (to me) and unique kind of blade. Too long to actually use all the maneuvers we see done with a longsword, but it still can do some. While being big enough to use many of the moves of a pole-arm, without having the often funky weight distribution of a pole-axe. They had a specific purpose, namely, destroy pike lines. But, interestingly we have records of them being used in late medieval/renaissance duels, where, they appear to perform actually pretty well. Having such a huge reach advantage will do that for you. But, it also means you have to carry that 6 ft tall monstrosity around with you all day, which no one wanted to do.
Anyway, yes, you put your hand on the ricasso. That's why it has the leather hand-hold grip, that's why they were developed with a secondary guard hooks. They don't add that stuff (particularly weight on the blade) for no reason. It serves a purpose, and that purpose is to allow increased control over this ridiculous 6 ft (sometimes even 7 ft) long swords that couldn't quite be controlled like a normal two-handed longsword anymore. It was an attempt to take the parts of both a sword and a pole-arm that you liked and mix them together, and, honestly, it was only somewhat successful in that attempt. But it was used, and it did serve a purpose.
The index over the guard being dangerous is kind of true, since, you know, all sword fighting is dangerous. But we have fechtbuchs of people doing that with arming swords. So, yeah, it was done. It's a maneuver that was used. We even see some arming swords that have little ring hooks to put your finger through if you wanted. They were never particularly popular, but they were done.
Though, yes, unarguably, when the fencing-style swords with baskethilts were developed finger over the guard became much safer and was more commonly used.