While it may not technically be rat holing, a lot of players do consider it to be just that. And even though most sites have measures, such as time restrictions, in place to prevent actual, blatant, rat holing, many players still frown upon it. And still, a lot of players do it. The way I look at it is, if it is within the rules of the site, who am I to question it?
My argument against it is that it takes time to learn and get used to a table, not just the players, but the whole dynamic of the table. Sometimes certain factors, such as your position at the table in relation to a particular player or players, can make all the difference in your winning or losing. If you've had success at a table, why would you want to leave a favorable dynamic for what could potentially be an unfavorable one? At the very least, it will take some time for you to learn the dynamic of a new table.
I won't say that I've never left a table to protect my winnings. But when I do, I'll usually take a break for a few hours. In fact, if my intention is to continue playing, what I'm more likely to do is to take my winnings to a higher stakes table. For instance I might take $150 off of a $50NL table and buy-in for $150 at a $200NL table.