A lot of people who are currently in the Merch by amazon program choose to not max out their uploads each day. For those of you who don't know how it works, Amazon gives you a set amount of uploads per day, and you can upload shirts up to that number. You are not penalized for uploading less. I feel that people who aren't using all of their uploads each day, are essentially shooting themselves in the foot.

You Don't Know When They Will Cut You Back

Amazon gives you more uploads; Then they cut you back. Less.. Then more... Less than more.
While we can predict when they'll have cut backs, Like before holidays. We don't know for sure when they will cut you back. So by taking advantage of your daily uploads, you don' thave to worry about "Missing out" on a potential sale because your designs are already uploaded.

If They Don't Sell You can Replace them

There is that awesome new rule that takes down shirts after like 90 days if they don't sell.
I am in love with this rule. Of course, you can reupload them.
This is great, Because why waste a slot on a shirt that doesn't sell When you can replace it with something that will sell? Maxing out your upload allows you to cycle through new shirts, New ideas, New keywords. If you haVen't already found a great niche, You can find one using this method.
People have made crazy amounts of money doing this!
And I've found that as I "Cycle Through" Shirts I gradually see my sales go up. Because let's be honest, My first shirts I made when I stared out are pretty awful! So if you are barley making your quota each day, Just make sure SOMETHING gets uploaded.

You Can Always Go Back And Edit

Did you know that for each shirt design Amazon only requires the Title, The upload, and The brand name?
Yep, So those keyword and description boxes can be left blank. And in some cases, I've done that when I've been in a hurry to upload all my 40 shirts by 1:00 A.m. (That's the cutoff time for me. )
You can edit those shirts the next day, Or later on. Might as well take advantage of your slots.

Sometimes Merch Won't Tier You Up Until You Use Your Slots

This seems to be a recent complaint lately. Those people who sell a lot of shirts hit their mark, Well before they are maxed out. And sometimes Amazon won't tier you up until you are maxed out.

It's partially a numbers game

I don't like it when People say that it's purely a numbers game. More shirts don't necessarily give you more sales; Sure it ups the chances though.
But when you have more shirts up, You have a better chance.
So if you have lots and lots of shirts up, You may get more sales. Especially during the holiday season.

You can always remove shirts later and upload better ones that will sell better.

Conclusion

So If Amazon gives you 100 uploads a day, Try and use them! It will allow you to scale your business faster.