To ensure your trusty picnic blanket doesn't get wet from the grass, try tossing a shower curtain in your bag as well. They're inexpensive and will make sure everyone goes home clean and more importantly dry!
If you're headed to a place that won't allow in chairs, a blanket is a great substitution, but after a full day of hanging out on one, the bottom can become stained or overly wet if the friendly patrons behind you spill their drinks. Keep a shower curtain handy to act as an additional layer of protection.
You could of course skip the blanket all together and just carry the plastic which in a pinch could also double as a killer poncho! Check out more ideas for reusing a shower curtain over at Woman's Day.