One "pro" for hole-y blankets: they keep the blankets from being too stiff and heavy. I find that even fairly hole-y blankets are still plenty warm, and the holes make them lighter and more pliable (for bunching up/squishing around and snuggling up with)
If you use acrylics, here are some of my observations: vanna's choice is by far the thickest of the worsted weights, and pound of love tends to be on the thinner end, with your normal red hearts and bernat super values in the middle.
It depends on the project, and how/when you're switching yarns. I've found that as long as I use the same yarn for an entire round/row of an afghan, it doesn't matter so much if different rows are slightly different in terms of yarn thickness.
Someone else may have a better suggestion, but if I were you I would just cut the yarn, cut out the knot, and just keep crocheting with your yarn, working in the ends either right away or later when you're finished.
I don't mind patterns that only have one picture. Unless the pattern includes a repeating sequence that might be better represented by a chart, simple text instructions are just fine and are very common.