If the United States required that all persons serve 1 term (2-4 years) in the Armed Forces, would that help or harm our society over time?

I would say it would be a good thing for the country IF, and only IF, there were no deferments allowed, except for people with disabilities, all 7 uniformed services took in personnel, not just the Army, and there were possible alternatives (such as serving in the Peace Corps, Reserves, National Guard, Americorps, etc.).
Too many of the privileged never serve, in fact, I think it's less than 2% of the population now that has ever served. There are too many people in this country that don't have a clue what civic duty means, they're into doing what's best for themselves, not helping their fellow citizens, etc. Politicians should be required to serve before beginning elected office, and I could go on and on. And I would make it equally apply to women as well.

Were this to come to pass, I see it as affecting our readiness negatively.
1) A vast majority of service age males couldn't pass the physical standards as it stands now. Its bad enough that the Army allows soldiers to graduate from Basic Training without ever meeting the minimum scores for the APFT. Soldiers only have to score 50 points per event and they will go on to AIT.
2) Economically, the US could not afford to have everyone serve. At some point, I could see Congress cutting pay in order to pay the salaries of all of those on active duty. Or they would raise taxes in order to pay for the additional manpower.
3) The turnover of soldiers would leave a vast majority of the services in a constant state of flux. In the mid to late 80's, the Army Cohort system brought in a battalion's worth of new soldiers who were together from day one. From personal experience, my platoon suffered an almost 67% attrition rate for the three year life cycle of the cohort we received in April 1989. Of the 23 soldiers we received that month, only 7 were still with the platoon on their contracted ETS date. I don't see mandatory service being any different.
4) Congress will still ensure there will be deferrals and you can bet their kids won't ever serve. They'll get out of service some way, some how.
Lastly, this, just like the talk of bringing back the draft is just designed to destroy our military readiness as you don't fix something that ain't broke. But that's just my jaded opinion.

I only want people serving that want to serve. This would be a bad idea and would do more harm than good. Our military could not handle that many people each year, and the cost would be enormous.