Yup, done that.
And while we could argue all day about whether that incident reflected my partner's lack of control, my lack of skill, or both, I'm just happy to still have a full set of intact, functioning fingers.
Actually, when this has happened to me, it was because we were training full out, with strong intention. It wasn't some lack of skill or control, it was two people pushing the envelope. The main reason most folks can get away with training with no tsuba is that they do not really go for it. If you are doing a per-arranged form in which everyone knows exactly what is going to happen, and the parties are trying to do the form rather than really hit each other, you can have a very controlled exercise and no ones hands get clobbered. Take it up a notch, with both partners looking for that instant of an opening for an entry and the hands will get hit occasionally. This whole argument is like that old one in the NHL. When I was young, hockey goalies thought that wearing a mask was un-manly... They had scars and missing teeth, and occasionally one of them would get a serious concussion, but they were tough boys, all. Finally, the League decided that putting your life at risk, just to be manly, when there was a perfectly good alternative available was just silly.
I feel the same way about tsuba. All real swords have them. Many koryu bokken and even shinai have them. Kendo shinai have them. They serve a protective function and also allow a whole category of technique to be done that you simply don't do if you don't have tsuba. A tsuba gives a sword an extra dimension, mainly for close in work. You don't have one, you change your technique based on that.
There's not some advantage to not having one, it doesn't make your training better... you are not a better person because you choose to keep your hands exposed to injury.
And please, if you are someone who maintains that you can hold a bokken anywhere, don't come to any seminars with Saotome Sensei... holding your bokken wrong, as if it's a stick and not a sword will illicit a lengthy tirade which the rest of us, who know better, will have to sit through. I was trained that one should treat a bokken as much like a live blade as possible and I teach that to my students. Live blades ALL have tsuba. A bokken without a tsuba is less like a live blade than one with a tsuba. So, we use tsuba. Saotome Sensei and virtually all the seniors who have trained under him all have bokken with tsuba. It is idiotic to maintain that not having a tsuba is some sort of virtue... it's a personal choice, one dictated by how one uses the sword. The way we use a sword, one wants a tsuba. Folks who come to train at our dojo who don't have tsuba, soon get them. They don't have to be told to... the advantages are self evident.