I won't deny you can fiat ways for characters to be useful(though I find having to fiat a character into usefulness to speak volumes for itself already). The problem is, in your average campaign, pretty much any spellcaster will beat out using anything else 99% of the time.
That hasn't been my experience, particularly if you make them keep track of spell components and such, for the spellcasters that need them.
Even if I was running a game for a group of fighters and spellcasters only, I would still throw in a lot of things that don't revolve around either skillset. I like for players to have challenges that fall outside their comfort zone. I don't really have any problem giving characters with special skills a chance to use those skills, either. As I mentioned earlier, I don't run modules, and I leave most of my adventures open-ended to accommodate things like that. Even though there are individual adventures, there is a strong sandbox component to a lot of them.
When I encounter players who want to play a character who is far outside the norm, I encourage them to do so. There is absolutely no reason that a character in one of my campaigns has to have spellcasting or combat abilities. I heavily reward roleplaying, and it doesn't ruin an adventure if the characters decide to not pursue the "bad guy," or decide to abandon their initial goals. I keep a lot of different encounters and adventures prepared, so if we get in a game that's set in the north and they decide to go south instead I can adapt to it on the fly.
From the player side, I once played a highly eccentric dirt farmer in a typical slash-and-cast campaign. He had no formal education, no real fighting skills, no spellcasting ability, and had no interest in treasure, magic items, weapons, etc. He primarily just liked to wander around, scratch in the dirt, and do some exploring. I created him as a challenge to myself, to see if I could play a character like that and still contribute to the campaign. The DM saw some potential in that and allowed it. The character ended up being a major driving force behind the adventure due to his tendency to focus and explore things that other characters had no interest in. The way I played him resulted in a character who was very good at rooting up obscure clues and bits of information, simply because he was nosy and didn't know the "proper way" to do such things. Like a child, he asked questions and played around with things that were considered inconsequential to most adults.
The campaigns I run are more akin to novels than video games. There is not always a single goal and a big bad guy to defeat at the end. Even when there is, the devil is in the details, and most details in my games have nothing to do with spellcasting or combat.
To use a comic book analogy, there is as much chance of Hawkeye gaining experience and being a contributor to the campaign as Iron Man or Thor.
