To echo points above, I'd always want to see it stated for clarity. If someone writes "5 Dark Reapers" I'd read that as unambiguously *not* including an exarch. I'd imagine at first that for reasons of fluff or handicap perhaps they'd decided not to take one.
If they wrote "5 Dark Reapers, 1 is Exarch..." or "5 Dark Reapers, includes Exarch..." then it'd be clear how many models total, completely clear what they were and would only take them a few seconds more when writing the list.
The reason I think those few seconds of clarification are important is that whilst they'd be redundant a fair percentage of the time, on those occasions when someone took issue, they'd save everybody a huge argument and potential ill feeling.
Not everyone is fluent with every army book of course. Whilst I'd get the logic of it being ridiculous not to have an exarch, if someone asked me to apply that to some other faction it'd all be a little tiresome. I rarely play any of the Imperial factions, and imagine they all have some extravagant US military/latin named upgrade that would follow the same logic. Trying to learn and juggle all that new information when about to start a game would waste so much more time than having everything in black and white to reference as and when needed.
Playing casually with friends it might not come up often (though I know it would be the legion of pedants I call friends), but in pick up games at GW stores, etc. when you're suddenly playing against people of all ages and experiences, I think it's more important. Imagine trying to explain it to a ten year old with mother in attendance as he thought he'd just killed your exarch thinking it a standard aspect warrior... "No that still has a wound left, it has two, because it is an exarch, no it doesn't say that on the paper, but logically it would have to be because it would be inefficient for it not to be, given it's the same point cost, but better..." The parent would just think you were pulling a fast one, and it'd be deeply uncomfortable. For me the few seconds of clarification that make sure that scenario, however unlikely, never comes to pass are a great investment!