The danger lies in the fact that instrument cables aren't made to pass the sort of voltage and current levels that an amp puts out at high volume. Pushed too hard and for too long, the insulation can break down, and then the cable shorts and kills the amp.
Your Markbass is exactly the sort of high-powered amp that engineers were thinking about when they issued those warnings. It's safest to stick strictly to Speakon cables for your cab(s), since that eliminates the possibility of accidentally grabbing the wrong cable out of your gig bag. If your Schroe doesn't have a Speakon jack, a tech can easily retrofit it.
Speakon connectors are also preferable because they're non-shorting. Disconnecting a 1/4" cable causes a momentary short each time it happens, and can eventually lead to amplifier damage if it happens while the amp is powered up.