Not sure about the seals wearing out. BUT-I've heard that leaving the slide out for an extended period of time can (but really? does it?) cause the weight of the slide to kind of start to droop/or sag. That's what I heard. BUT-On a small to medium FunFinder I'm not convinced the weight of the slide will drop. Maybe on a huge fifth wheel where the slide is super heavy with appliances or a sofa and the dining table and chairs. . . So - having heard what I've heard. . . I store mine (indoors) with the slide out. Some may say that with the slide out in extreme cold or torrential downpour, the seal(s) could leak. . . maybe.
2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW
We store our unit with slide in. Though it is under a roof, I feel it is an invitation for insects and critters. Even if stored outside why would you want to chance developing a leak in addition to insects and critters.
Store the trailer with the slide in. Some models require the slide to be out to have full access to the trailer. So you may see people storing it in the out position so they can perform maintenance or upgrades in the off season. Or like a neighbor of mine, they use it as a guest room for when their son visits.