If mcat is a strong point for you, then it'll probably be fine. If not, taking it the first time in Aug & applying the same cycle is risky. You might end up losing your app fees if you get your scores back & then decide not to continue applying this year.
From my experience doing that last year, here are the drawbacks:
1) while folks are working on their 2ndaries, you'll be studying for the mcat. I hardly had enough time to study, let alone keep up w/ 2ndary essays last summer.
2) it was torture to have to wait so long for interviews! I got my first interview in Nov (not scheduled til Dec), while others apps who didn't have the aug disadvantage & applied early were soon to be receiving acceptances in Dec.
3) THE LONG WAIT post-interview!
4) West coast schools take their good 'ole time, so it wasn't too bad, but I felt the aug mcat put me @ a disadvantage @ east coast schools, which move much faster through the cycle.
Depends on your own situation, schedule, finances. If you're younger than 25, then I'd say take your time to build up a strong app, work on essays & apply next year w/o the Aug disadvantage. For me, it was like the "curse of the Aug mcat"!