If you really try to put it into practice boarding will take longer whilst you attempt to organise it, and organise the shown seats!
In my experience it does work. However not for every clientele. In my outfit we do use block boarding (boarding group boarding as we call it) and it has sped up boarding somewhat. However primarily on flights with a high percentage of business/frequent travellers. On flights with holiday travellers it takes more time at the gate to explain as passengers are unfamiliar with the process, although they tend to follow rules better than the business crowd. Some generalization here of course.
It puts a higher stress on the gate staff that has to organize it, put passengers further back in the queue until their group is up for boarding and do more boarding announcements. And of course they need to be trained in the procedures and the differences in boarding via a gate or bus/2 stairs.
Now, i have flown 737s with sliding seats, however those were used to convert a 3-3 configuration into a 2-3 business configuration with wider seats. It worked quite well, but needed some time and we had maintenance doing it as it took some strength to slide the seats in on one side and out on the other.