Nope. If the s'chach isn't valid, it's not a sukkah.
The practical advantage is, you can have all sorts of nice stuff in the sukkah; if it starts to rain, just put up the tarp; as soon as it stops raining you can pull off the tarp and get right back to your mitzva. Whereas if you didn't have the tarp, you have to rush everything out of your sukkah when it starts to rain; then when the rain stops, you have to wait another hour+ for everything to dry out, then drag everything back into the sukkah. (Practically, people are likely to not bring out their best dishes, chairs etc. anyhow because of this.)