I personally think it would be nice if Metro bent the rules a bit. Even Pinstar allows certain mods in his legacy challenge as far as I know. A lot of challenge creators have the same basic rule, as to what we think it should be: "Mods are fine as long as they don't give you an advantage."
Yes, it would be nice, but it's just nice to have a challenge in TS3 that actually challenges me. That I might fail it. And not because I got bored with it. Yet is doable and with different strategies as noted in several documented stories.
But Pinstar isn't Metro and Pinstar has the legacy be for everyone. Even those not really looking for a true challenge. And Metro makes his challenges only for those looking for a true challenge.
Being forced to play with no mods to go for Hall of Fame (and I normally don't go for scoring) made me realize that oh, EA fixed some of the problems that had me playing with mods. I have to admit that I was very leery of what kind of story progression my game was going to have. Beyond some playing way back before World Adventures was even released, I had never played without a mod affecting Story Progression. And all I remember is not being impressed.
Also with Overwatch deleting 50+ cars each night I was truly afraid my game was going to crawl to a screeching halt because of severe lag. Turns out it didn't. Or at least not yet.
Plus, with at least one file where I don't have mods in, will quickly let me judge where an issue is mod related or glitch related. Even if I wish I could have a few primarily bug fixing mods in there, and would probably put them in if Metro allowed it, the lack of mods haven't adversely affected my enjoyment of the game.
So while I would love for them be allowed, hence why I thirded the initial question, I completely understand the reasoning behind the rule.
And jeana, I agree, every time I go to post, someone else has posted. Or one post, 23 posts had been made, but I kept getting distracted while writing that one.