Every season for boat trailers!
How else would you jack it up? Also, not sure if I'm telling you something you already know, but putting grease into zerks is not greasing the bearings if it's something like bearing buddies. You need to actually pull the inner and outer bearings periodically and grease them by hand otherwise you'll end up stranded on the side of the road.
I worked in a shop for a few years and I can't tell you how many people towed their busted down trailers in because they relied on bearing buddies.
You are spot on man! For trailers that get dunked in water every season, you need to do a full bearing pack. If you can't do it, pay someone. We charge 65 bucks plus the cost of the seals at my shop, but it is really an easy thing to do. I helped a friend last night pack and replace some bearings on an old boat trailer he just purchased. It is a really good skill to learn, because you will be able to get yourself off the side of the road if you have a bearing fail.
Since most of our trailers don't have brakes, it is not a concern. But for braking axle trailers, any bearing buddy, or dexter EZ lube axle gives you the ability to pump grease into the hub until the hub fills up with grease and blows the seal out inside the brake drum. Then the grease contaminates the shoes, and you need new trailer brakes. Trailer brakes should be visually inspected regularly anyways, so I don't understand why Dexter even uses that setup. You can blow the seal out just as easy with a no brake (idler hub) axle, but the grease just gets on the wheel and trailer, not really ruining anything, just making a mess.
I always find water or the affects of water on trailer hubs when I perform bearing packs. Even when you add more grease, it doesn't eliminate the moisture in the bearings and hubs. Also, there is specific grease you use called marine grease, instead of standard hi temp grease. It is thicker and is sticky like peanut butter.