It's not that it's "better", but rather my preference. Less sheen means less reflection. There's really no reason for any gloss when the art is already covered by a gloss sleeve. There are great options in gloss and semi-gloss/satin as well.
http://www.adorama.com/searchsite/default.aspx?searchinfo=epson+ultra+premium+satinDon't let the whole cutting thing bog you down. You're already cutting the covers once printed, so what's one more cut per print gonna hurt?

Plus, you get way more covers for your buck when you purchase this way. Check the math on what I posted versus some middle of the road legal-sized stuff. The super nice stuff almost always ends up being actually cheaper than the legal sized stuff. Not to mention that there are NO top-notch photo papers in legal size because it's, well,
legal size. Won't find a lot of artists and photographers looking to print on legal size

Here's the quick and dirty when deciding between gloss and matte.
Matte = richer colors & crisper images
Gloss = darker blacks & lots of coating options that make the artwork a lot more durable