The effects will be higher turnover and more people may participate.
I originally joined on a 2 x 6 program (which is no longer available). 2 years active duty, 6 years inactive (non drilling). I did not get A school training, but received a short indoctrination school to prepare me for the fleet. I 2 years, I completed all of my requirements for 3rd class and passed the test. I was frocked to AE3 the day I went on leave to get out (I did not re-enlist because I wanted to go to college full time). I'm still working on my degree (I got side tracked by life

, but have re-enlisted in the reserves.
I'm not sure I would have joined if I didn't have the 2 year option. There are a lot of uncertainties for kids just out of high school.
The good thing about the program was I received VA benefits from the state and GI Bill. GI Bill is at a slightly reduced rate, but it still helps.
I like the program and see little negative. As long as the 4 and 6 year enlistments are available for those who want the advanced schools, I see no problem letting people in who understand they will not receive the same A and C school training as longer enlistments.
I don't see it a good idea to offer those other schools to people who only sign up for 2 years. It would be hard for the military to get their moneys worth.