If a poncho has grommets or tie strings at the corners and maybe at a few spots along the edges, it will make a perfectly fine shelter.... Do carry assorted lengths of cordage and learn your knots.
I use my ponchos for shelters and love the convenience of that. (It took a little while to get used to not sleeping in a tent and now I have a hard time the other way around. LOL!) Grommets are great, and better quality ponchos seem to have more (and better) of them, but you can do without. Just use a rock as a button, and loop your tie-out around it. Works great on blown-out grommets too!

As hikermor says, bring cordage and know your knots!