How about instead of ad hominem attacks you address the actual argument?
Assuming you go to mage classes at level 20 (any earlier won't really help) you'll be at mag 17 at level 20 with the minimum stats for mage classes. By level 39 (I just chose that because that's where I am right now) your magic should be at about 23. With Thoron (his best spell outside of aura which is too limited in uses) your atk is at 32. Adding 3 for fiendish blow (3 instead of 6 to average out between player and enemy phases) you're at 35. With tomefaire, 40. Also, since this is a mag class, you don't have access to brave/killer weapons, no offensive combat skills, and your crest is now worthless. I guess you could theoretically use magic weapons, but that seems to defeat the whole purpose.
On the map I'm at right now (ch 20) this would mean that you are offensively weaker than 33 out of the 42 enemies on the stage. If we ignore the healers, then 31 out of 40. In pretty much all cases, you're not just weaker, but weaker by at least 10 dmg. And most of those enemies you do have a higher attack than compensate by using death, which at least has a good crit rate.
The game is designed around the idea that your offensive units (and I doubt you'd be using a mage Dimitri as anything else since he gets terrible support spells and is not bulky enough in that class to be used as a tank) are going to be stronger than the enemies. That's why they outnumber you by about 4 to 1. I'd say we can objectively consider classify any unit who is so much weaker than the enemies (at recommended level) as bad. Especially considering how useless they are compared to what they would be in another class, or if replaced by another unit.
These, btw, are those enemies that you've complained are so weak that it makes the game completely devoid of challenge. Considering that, I'm not sure how you can argue that Dimitri's magic would be "decent" by even the loosest definition of the word. Please define what you mean by decent.
As for, "not the best", it's not just not the best, it is actually the worst option by far. If you don't think so, then please let me know what worse class you could choose for Dimitri. If we want to be stupidly literal, then worst does fall under not the best, but if you say not the best when you mean the worst, then you suck at using language to convey your thoughts intelligibly, because that is not how anyone uses that phrase.
How about you read what I said? I'm not interesting in making walls of text over something dumb n pointless, especially when you're taking it way too seriously and aggressively. You're just gonna have to accept that.
I will get facts straight however, namely your incorrect claim that magic classes can't use brave/killer weapons. Any class can use any physical weapon provided they have the weapon rank for it (mounted classes can dismount to use fist). It's only magic that's class specific. Once learned skills can also be used on any class.