Hey, I'll back up Rambo 100%. A cake tester is an much needed tool in the chef's arsenal, IMO. I use them all the time.
On the hot line, obviously, we don't use them for cakes. It's a very simple and accurate method. You get a great cross section of the temperature of the meat. Like Rambo said, if it is cold in the middle, well, still RAW. Cool to blood temp in the middle, rare. Slightly warm in the middle, mid rare. Warm, medium, etc.
Really guys, it works great. I couldn't imagine working a station without one. It can also be used to test whether food is hot or not. Re-heating a braised short rib? Stick the tester in, wait a few seconds, and test it. Easy way to tell. With something like that, all the "touch" in the world can't tell you if it is hot in the center. Maybe a good squeeze will tell you if it is still hard in the middle, but a cake tester is much more accurate and removes all doubt.
I'm an advocate of using both touch and cake testers. I mean, touch is great, but every steak is slightly different. Some Filets have a looser grain, or the meat is softer/mushier. These steaks temp different than a "normal" filet. Some of these steaks feel MR when they are closer to MED. Touch works well and has it's place, but so does a tester.
Works great for fish too. Fried things? Hell yeah. Is my big arancini hot in the middle? I dunno, let me probe it.
Moisture/juice loss is EXTREMELY minimal. Never, ever, ever been a problem. I would argue that the minimal amount of liquid lost (and, again, it really is very very little, if any at all) is of much less concern than over/under cooking a protein and having to re-fire or even replace it. A properly cooked, well rested steak is the goal. I mean, the probe is probably a little less than the width of a paper clip if you unwound it...very thin. I've done this quite literally thousands of times and have never seen a gush of liquid come out of the tiny hole I made after I probed it. In fact, a rare/MR steak let out no discernible liquid, and the only bits I might see on a MED/MW steak is a few drops of red blood/juice form around the hole. Like I said, I've never seen more than a few drops of liquid come out, at most.
A cake tester isn't nearly as thick as a insta-read probe. I know that when you stick a probe in meat like that a lot of juice come out of the hole (we've all done it on a roast or turkey I'm sure) but I'm telling you guys, a cake tester doesn't do it.
Lol, actually, at my current job that I started like a year and a half ago, I walked in on my first day and unpacked all my stuff. The other cooks wondered why I had a cake tester with me...thought it was stupid. They didn't get it--but man, I was on fish station and I couldn't imagine working fish without one (it's how I learned). I continued to use mine, taking my lumps from the guys. Slowly but surely, over the course of a year or so, cooks would ask to borrow my cake tester for various things. A few months ago, we had a new head chef start, and he brought his sous with him. They wondered why I was the only cook in the kitchen with a cake tester...now it's standard issue for everyone on the line. Kinda felt like a trailblazer, and I felt a bit vindicated in the end.
Just kinda funny to me that they started out thinking it was stupid, but now see the light.
Rambo, keep it up!
I will also add that it seems to be a high end fine dining technique/method, and I don't know how widespread it is. But in like 4 of the 5 kitchens I've ever worked in, we used testers. Also, words like "jabbing" and comparing them to "nails" is pretty silly. I mean, that video that SquirrelRJ posted featured one of the best chefs in the world, so take that how you will.