The reason I would say something like that, is out of respect for the country, the flag, and those who fight to protect it.
This is interesting because the Pledge issue does get people wound up and emotional. I would advise for anyone who has a strong opinion either way on the issue to look up Francis Bellamy as his history is not what many people would think.
The constitution was about ideas of liberty. The "flag" and the "republic" are offsprings of the constitution.
The intersting question is what would happen if the "flag" and the "republic" one day represented a government similar to that of the United Kingdom? How many would still have strong views in favor of it if one day the United States looked more like Europe because setting the 2nd amendment aside, the United States is looking more and more like Europe every day?
In my view, someone saying the pledge who has no respect for the Bill of Rights should be held in a lower regard than someone who pushes for liberty (MOC activism, FIJA etc) and doesn't say the pledge.
Personally, I do say the pledge if I'm at a sporting event or somewhere where it is a custom; however, at the same time, it doesn't define me, I have no strong views on it either way, I understand the history of it, I realize that the United States survived for 116 years without it, and I accept that the author of it was a devout socialist who had little respect for the Constitution or liberty in general.
