It's that the gaps tend to be odd. Which means that they end abruptly and without notice. It can be quite awkward to come back and read the last sentence -- or even that you broke off in mid-sentence -- and wonder quite what were you thinking.
Working on your own time, if the work gets up a good head of steam, can eat up the hours and churn out a lot of wordage. It's a lot easier, however, to stop at a good breaking point, even if you suddenly realize that it's really, really, really late, and you need to get up in the morning.
Still lug about the stuff. It's not like bringing a book to read wouldn't lead to the same awkwardness of time, and it does get written.