The reasoning is the color will already be on the wall, just need a sheen difference. A thin even coat of poly or some other clear coat will be easy to apply and it won't take very much of it. Another consideration is when using another can of paint. There is a good chance that the new paint for the stripe could be a slightly different color. This would make the stripe really stand out.
I would use an <a href="//www.painters-toys.com/deft-interior-acrylic-wood-finish-satin.html" target="_blank">acrylic urethane</a> with the sheen of your choosing. The acrylic urethane is totally clear, won't add an amber color as with oil based finishes, plus if you don't like it you could simply paint over it without having to prime first.