No, it doesn't contradict/counter the argument that violent rebellion was justified because of abusive and oppressive behavior by the Templars. SOME mages who are lucky enough to live in a non-barbaric Circle get a chance to use their magic for profitable
artistic enterprises (especially if they're well-connected Lucrosians). S-O-M-E does not mean all, and you can hardly use this letter to extrapolate that all mages have nice, cushy lives.
This story is an exception, rather than the rule. Stories like Cole's (and I mean the original Cole) about physical abuse and maltreatment are probably more common, and I'm fairly certain that there were letters in the game making it clear that some Templars did act like monsters.
Not all Templars are monsters, but not all mages live a sweet life like you're trying to claim. Also, I think the author was pointing out that the non-mellow, whingy Templar who has his panties in a bunch over mages using their magic for artistic enterprises and making money
at it needs to chill out because they're not just doing something frivolous. Instead of using their powers solely for destruction - something that presumably doesn't take that much skill or control - they're doing things that require a great deal of skill and control that either result in works of great beauty or enrich people's lives.
You claim abuses are more common but present no argument supporting it. It is merely what you think is probably more likely to occur.
However, the simple fact that Circle exist are evidence they are making money for what they earn is what sustains them.
In fact, even Kirkwall's Circle, which we can agree was the worst to be stuck in, had mages and Tranquils regularly selling wares with one even remarking how Orsino had been increasing prices.
In fact, we could analyze all the Circles we have seen or heard from.
There's Ferelden which was a good Circle where the KC and FE were even long time friends.
There's Kirkwall which was bad, especially when Meredith was corrupted by Red Lyrium but still seemingly preferable to being in Darktown, never mind Lowtown.
There's Starkhaven's which we know little about but do know that, according to Alain, it's different from Kirkwall's because there, Templars beat mages and nobody said a thing. Meaning, in Starkhaven, either Templars didn't beat mages or, if they did, they could expect sanctions.
There's those in Orlais were mages were allowed the run of the tower, could visit the city they were housed in periodically and there were even dozens of troupes specialized in bringing spectacles to the towers. This before Anders, of course.
There's Nevarra's where we know little of the Circles but do know that the Mortalitasi are very influential and wealthy.
And there's Rivain's which could hardly be considered a Circle at all.
So, out of all of that, the only one that could be considered to be ghastly, and even then, it's Kirkwall's which was a rather special case, both because the Templars ran the city, the Veil had more holes in it than Swiss cheese and the Knight Commander had her mind corrupted by the Blight.
Can abuses happen in any Circle? Sure, just like the guards can abuse townspeople anywhere too. For the most part, the Circle clearly provide a quality of life above that of the average Thedosian. Food, shelted, money, education, accomodations, etc.
So, the question becomes, should we endanger the lives of people who already lead harder existence than mages because of some abuses? Mages already have so many advantages and we would add more?