If my draw Length is 28 is there a reason I couldnt use a bow with a 29"

Yes. You would have to shoot high wrist or over draw on your length and that would seriously affect your form. You might be able to get some twist put in the string to take up some of the difference but without a solid back wall and good form it will have a negative effect on your consistency.

Is there a reason you cant? no. there is no reason you CANT.

You should not because you will more than likely have bad form, you will more than likely slap the crap out of your forearm every shot. you will not have a solid back wall to stabilize your shots on a constant basis. your anchor point will more than likely change with every shot due to not having a back wall. you will creep on your shots and be very inconsistent with arrow placement.

Now with all that being said. You can create a form that will work and you can become good with it but it will take time and practice. over all it is just better to get what fits. Kind of like pants. get a pair to big and there is a lot of room for error.

Be prepared to get got by the string

And that sucks

The only way I can say it is why would you wear a size 10 shoe if you wear a 9... Form is everything. that is draw length just go with it. I'm right in the middle of 29 and 30 but go with 29 to keep the form right. I can shoot a 30 but I like to keep a slight bend on my arm and my follow through is so much better since I shoot back tension release on targets and thumb for hunting, both releases give me that slight push that allows the follow through to work smooth.