I've never owned one, but I have checked it out in one of their stores. Personally, I wouldn't see a need to double up on guylines on just about any decent tent, and REI definitely isn't Ozark Trails. On the Basecamps, the cross pole design along with the sleeves on top and corners make for one helluva sturdy tent compared to tents that are just clipped to the poles. The 11mm main poles are no slouch either.
I just looked at the reviews on the REI site and a couple of people did mention that they bought extra tieouts. But, aside from the usual overly picky people slamming the tent, only one seems to have had a problem in a storm - an 80 mile an hour thunderstorm that destroyed it. In my mind, by the time the winds are rattling a tent to where you think it might collapse, it's time to bite the bullet and get soaked while throwing everything in the car and beat feet. Unless you're in something like a $900 Hilleberg, some storms just aren't for riding out no matter how many lines you have pegged down.
All that to say, I would spend a few bucks to get some extra guylines, but I wouldn't try to go overboard with 'em.
