I recently started using paint brushes for glue... I had heard someone mention the precision and control they had using white glue diluted with water, and a brush--- not just control over amount and application, but they referred to a kind of constant control over the amount of water in the glue, and therefore the thickness and dry time. Well, I found a small device that I suppose is for painters to clip on to whatever they hold in their lap--- for me, a clipboard with a cutting mat on it is what I hold in my lap, and I found this doo-dad clips onto it perfectly... it has two metal cups on it, and I put some glue in one, and water in the other... to add water to the glue, I just dip the brush in the water and wipe it off on the edge of the glue cup, as you would with excess paint.
Lo and behold, I found that the modeler I heard discuss these things was right on the money--- there is that kind of constant control and choice going on... so like you, Revell-fan, mein freund, I would prefer to use traditional watercolors because I'm already into the brush thing, and I can see the same choice-and-control bit really applying to the watercolors; even more than it does with the glue.
I can see why some would like the pencil versions, and I have noticed those before... but some innovations like that strike me as offering a solution for a problem I have not yet seen. Some of the glue pens kind of strike me that way--- it's like they're saying, "Finally, precision and control with glue application," and I'm thinking, "What, did we not have that before?" I sometimes apply glue directly from the bottle, and I don't think it's that imprecise.
One innovative take on a familiar product that I do think is a great improvement is a particular CA glue. Krazy-glue offers CA glue in a form that is liquid, with a brush in the cap--- similar to a nail-polish brush, I suppose. They also slightly tint the glue purple--- but the tint disappears on curing, so the glue still dries clear! For me, these two features have really helped with the two things I sometimes had trouble with using CA glues--- controlling how much I applied, and (related problem) seeing it well enough to know it was on there, or how much was. The tinting is so light it's still not obvious where I put it, but I can see it a little, at least--- before the only way I could see it at all was to look for the shine of wetness on the paper.