Firstly MF don't "recommend" that the amp be left on indefinitely they say "The X-CAN V3 has been designed so that it may be left on indefinitely"
So, they are not suggesting it "must" be left on indefinitely they are simply saying it "may" be left on indefinitely if so desired.
I know people who have had their V2's running powered up continuously for years without problems though you have to realise that "heat" is the biggest enemy inside the V2 and heat is only produced when the amp is switched "on".
The valves don't get too hot but my god the transistors certainly do and the ambient temperature inside the V2 is pretty high indeed. Consider that the stock Jamicon general purpose 85c caps have a lifetime of 1000hrs at maximum operating temperature,
have a read of this, and then ask yourself if it's good policy to leave the amp powered up (warm) 24/7 365.
It's a fine balancing act between keeping the amp on all the time to preserve valve life or switching it off when your not using it to preserve the in tolerance lifespan of the capacitors.
My advice, if you're considering leaving the amp on all the time, would be to replace all of the capacitors with Panasonic FM 105C varieties. You don't
have to replace the 85c caps but they will go out of spec a lot faster than 105C types in this particular application.