Thank you very much, David,
I feel I also have to thank Gustavo! And Hassan Hussain !
Gustave definitely has made a very good point.
I am going to try out a couple of new sentences,
What do you think of:
a) It is pleasant drinking a can of beer after a long day's work.
b) It is nice sitting by the river in the evening, watching the sun set.
I am not sure about these two. Maybe when the participle clause is very long this structure becomes awkward. That's a theory I am testing, which if correct, would complement Gustavo's theory (which is obviously correct).
I suspect that my theory is wrong and (a) and (b) are correct grammatically, but I am not entirely comfortable with them. One minute, they seem fine, the next minute, they look wrong. (Duck/rabbit)
PS. Sentences 5-7 would not be incorrect with a comma, would they?
Sorry about this!