A Reverse Question:
If there are no reason why SMN should not go melee in EVERY encounter ever, so can i assume you support SMN to be a melee class?
Every MMO out there have range and melee DPS, and with a reason they are classified as such.
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The reason is that they can attack from a range if they so choose.
There is no dead-range.
There is nothing forcing you to fight at range.
You function the same at range as you do in melee.
If a range can gain DPS by being in melee(for instance BRD weaving in Replling Shot), they should. There's no detriment to doing so, and it takes no extra effort.
Edit:
"Oh, I'm classified as a RANGED DPS so I have to stay RANGED." doesn't cut it when you aren't forced to be at range. When all of your abilities can function in melee, and some even require melee, there is no reason not to use them. None. There's no reason not to be in melee for a Miasma II cast every minute. The only time when this may not apply is when actual fight mechanics require a certain number of ranged(think the tidal surges or whatever from Akylios in Rift).
also, I don't ALWAYS try to run in just to get a miasma II right before contagion, example: I'm too far and it would cost me point something seconds of not doing anything if I go near just to miasma II or that if the boss has some kind of anti-melee attack at the start of the fight, the point is, why not do it? it's free dps increase, it's double the ruin damage + a book attack cause it's an instant attack spell.
// so can i assume you support SMN to be a melee class?//
I don't understand how you got to this point just because of miasma II. I would trade not having thunder and raging strikes if ever they think SMN should be melee.