These grains were cracked on March 19th

Would you still brew with them?

There are Carastan 30-37 and I've stored them in a bag, in a ziploc, in the freezer.

I've read that cracked grains should be used within a few weeks.

Yes, use 'em. Just make sure they didn't get freezer burn or anything like that. Let them warm up and taste them. Go ahead and crunch a few. Taste good? Use 'em.

I know the 'Brewer's Best' kits that my local homebrew shop sells come with cracked grains sometimes and they work well after sitting for a while on the store shelves. They are in airtight bags and if you sealed yours well and stored them right I figure they will be fine.

as long as they didn't get hit with any humidity you are good to go.

what is the issue with using grains that have been cracked a while ago? what does it do (or not do) to the beer?

When you crack the husk you release the fragrance and let the air into the innards of the malt. Kind of like if you cut a loaf of bread - what happens to the exposed sides... stale and not so good. Malt is not so timid as bread, obviously, but if left to the open air for long periods of time it will stale, especially if the air has high moisture content.

I have used grains that were cracked 6 months earlier, but kept in vacuum seals bags or airtight containers, with great results.

If they are a little "stale" just toast them on a cookie sheet in the oven (400) for a short time. I've done that before with good results, as long as their not rancid... I doubt these would be. FWIW... Storing grains in the freezer isn't the best way to preserve them. Cool & Dry and airtight.