I had one with my '08 210WBS. I adjusted it for the proper spacing and then taped it so that it wouldn't change during use. I laid it on top of the slide and I used it religiously. I also put a big neon red card, attached by string, on it that hung down prominantly in plain sight so that we wouldn't accidentally try to put the slide with it in place.
Now, that being said, do you really need it? A lot, if not most, of the newer units come from the factory without one. A lot of folks that have them never use them. I used mine. My thinking was that they wouldn't have spent the money on it if there wasn't a reason for it. Did it ever serve its purpose (to prevent the slide from coming out while traveling)? Not that I could tell. After a day of traveling it came out as easy as it dropped into place; the slide never budged and it never budged in the 38,000 miles of travel we put on it. I still used it as it only took a second to drop it onto the top of the slide, was easy to store next to the "couch" and came with the trailer (free, sort of).
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)