I use the mat markings primarily to line up the fabric fold when cutting strips. If you want your mat to last, it's a good idea not to use the mat markings for cutting because you end up cutting on the same lines over and over again, making grooves in your mat. By using rulers to square up and cut, your cuts are much more random all over the mat and not running right on top of one another.
The best cutting accuracy comes from using rulers that have fine lines, and from using only one ruler (or one brand of ruler) throughout a cutting project. Most of the quilts I make do not demand such high levels of accuracy, so I often combine ruler brands for a project. For miniature piecing or blocks that require a high level of accuracy to turn out right (such as snail's trail), I would be much more careful about which rulers I am using.
Regarding the Slidelock, I really don't have any problem using another ruler to measure. It doesn't seem like an extra step to me, but then I am used to doing it that way.
There is another way I use the Slidelock which may appeal more to you. I sometimes use my regular rulers to line up my cutting line and then place the Slidelock on top of the ruler before cutting. In this case, I am still cutting along the edge of my regular ruler, but the Slidelock is holding that ruler in place while I cut. This is particularly useful with my long rulers, which are the ones most likely to slip on me a bit towards the end. The Slidelock makes it almost impossible for the ruler under it to move. They do not demo using the Slidelock in this way, probably to avoid possible liability issues, but it works very well. The two things I appreciate about the Slidelock are: (1) it gets my left hand up and away from the cutter, and (2) it holds whatever is under it (fabric or another ruler) firmly in place.
I am in the middle of piecing a quilt so haven't done any cutting yet on the Martelli mat. I do have to do some cutting on my Olfa rotating mat, and I must say I have come to appreciate that surface after comparing it to the Martelli and Quilter's Select. The Olfa has a surface that allows fabric to be pushed around easily, just like the other mats. However, it "grabs" fabric and ruler better than the other two, plus I have now noticed that it does not require much pressure on my cutter to make a clean cut. My old no-name mat required more pressure on the cutter, as did the QS mat when I used it. I suspect the Martelli mat will also require more pressure than the Olfa. Once I get through my current project, I will be able to test out long cuts on the Martelli.