my experience.... fwiw
I have a 2017 F3L (with rear top case/side bags). At time of purchase, I had the dealer order me a BRP trailering cover, which I thought I'd likely need since I planned to take the Spyder south from Ontario to Florida each winter, and make it a year-round toy. I had already picked up a used, light-weight, open aluminum trailer even before buying the F3. Then, I read many reports warning against using covers at all while trailering, due to flapping and scratching of finish on the bike. So, I wound up driving south with a naked Spyder; we survived a torrential rain without problems, and arrived dusty but in otherwise good shape. More about trailering experiences with that BRP cover later.
All was not lost upon arrival in FL, as I had only a single carport, and it was already being fought over by two cars. The Spyder was relegated to a side patch previously used by a golf cart. That left it sitting in the blazing hot FL sun all day and unprotected from moisture at night; I was concerned about likely fading due to full UV rays. The trailering cover was put to use. However, it's bulky and a bit of a hassle putting on and taking off for each ryde. I wanted to have something lighter, simpler, less bulky that I could also take on travels, if desired. I bought an Ultra Gard lightweight cover 4-475AB, really meant for an RT Limited, but hoping it would fit my F3L. It did, but barely - the F3L top case is longer, so the rear of that cover was stretched to its very limit, leaving the windshield sloppily covered because the fit there wasn't right. I had also installed an F4 23" windscreen, which meant that using the BRP trailering cover was nearly impossible because of its greater width (the extension pocket simply can't cope with the wider F4 shield). But, the sloppy fit of the Ultra Gard cover also meant rain water would pool rather than run off. I switched windscreens (to a BRP 'Tall Boy') and once again used the BRP trailering cover.
Still looking for a light-weight cover, I did investigate the Nelson Rigg half-cover; I rejected it as not being sufficient to block hot FL sun on the bottom half of the F3L. Yes, it would have protected my seats, top case and windscreen, etc., but still leave frunk, front fenders and side bags exposed. I opted instead for their CAS 370, a very nice light-weight full cover, not too bulky and much easier to put on/take off than the BRP, with lots of extra room at the windscreen to accommodate various sizes of shield. The only area of the 370 that doesn't quite fit is the hole for the antenna. This cover was really first designed for the RTL; it's also big enough to adapt to cover the longer F3L top case. The antenna position on an RTL is about 4" more forward than on an F3L. Easy fix; just cut a new hole and reinforce with a few stitches near the convenient seam corner joint about 4" further back - makes the 370 cover hang/fit better all the way around.
So, FWIW, I like having the added benefit of a full light-weight cover, but I would certainly go with nothing less than a half-cover. Better something than nothing if your ryde is going to spend extended periods of time out of doors and otherwise unprotected.
A post-script on the BRP trailering cover; you'll recall that I said I didn't use it heading south last fall. This spring, however, facing snow and ice when headed north in April, I did use it... all the way up. I fastened it down tight to avoid excess flapping, and I checked my straps every few hundred miles. I did not see any exaggerated flapping along the way. However, when inspecting the F3L upon arrival, I did find damage to the finish. No 'scratching' per se, but rather small 'worn spots' rubbed bare of paint at the corners of my side bags, and in a couple of other areas where there would have been intense friction from the trailering cover constantly jiggling and rubbing against the most protruding points of plastic paneling, over which the BRP cover had been stretched. This happened in spite of those areas of that cover being 'protected' by an inner layer of soft material. Bottom line: I don't believe I'll ever be using (or recommending) a trailering cover on my north-south journeys again. The Spyder and I will just have to grin and bear what Mother Nature throws at us, and have a 'clean up' bath on arrival.
Hope this helps.