The purpose of all of these plot points is to limit Harry -- that's not a defect, that's an express purpose, both to keep the stories interesting, and to emulate the old-school noir fiction that DF takes so much inspiration from. Isolating and handicapping your protagonist is a standard for just about any genre -- so that he will have to be strong and clever to overcome those limits.
Off the top of my head I have two:
1) As discussed in another thread, I find the idea that black magic inherently corrupts a person (and as such the Blackstaff is needed for the White Council's dirty jobs) a poor choice. I think the idea works in Molly's disfavor and is honestly less interesting than the idea that such corruption is based on the person using the magic and not vice versa.
It's an intentional limiter. It's a way to force Harry to not blast everything that attacks him. It lets mafia goons stay a significant threat, and makes Harry and others have to measure their actions. If Harry wasn't worried about being corrupted or having the White Council after him, what's stopping him from just fire-blasting every mortal goon who gives him trouble? By making this a limitation, it forces Harry to grow as a character and keeps things interesting without having to go the DBZ route of constantly making everything he faces stronger and stronger.
It's also a way to introduce internal struggle, and create a very
real risk to the character. If it was just a White Council prohibition, well, they're not watching him all the time -- he could get away with it
constantly, as could Molly. But making it corrupting -- and having Harry already touched by that corruption -- gives him and later Molly something to really struggle with, something that informs and shapes their characters in a way that it just being prohibited would not.
2) Wizards inherently have issues with technology. To be fair, this was presented in a pretty silly way from the start. Harry talks about how he naturally can cause issues with firearms which is ridiculous (although the books do drop this as time goes on). Like I can understand how Wizards can screw with electronics, especially delicate things like computers (as it is computers in our world can have issues with outside energy sources). But guns are pretty simply machines, even the fancy one's. It'd be like if being a wizard meant you'd have cause a bicycle to fail. I could understand that a wizard could, if they try, bend the laws of nature. I mean that's what they do. But to inherently prevent things like combustion or lever or gear or springs (for example) from working is just silly.
As other people have pointed out, it's not a simple electrical-field thing.
That said, for actual tech like cell phones and computers, I feel that wizards being unable to use them is kind of meaningless for the story. Like, the White Council could still be behind the times simply because institutions are slow to change plus most of its members are pretty old (so no one would have thought of Paranet before Harry did). Similarly, Harry can still be on his own early on because for instance no one can get to him in time or he's purposefully keeping them out (which is something I did not care for, but it seemed like Jim intended for it to be a flaw of Harry's so its not an issue).
Meaningless? Far from it. It informs the entire wizarding world of the books and is a reason for why it's shaped the way it is. It's an ongoing, tangible issue that the Wizards' problem with tech is a significant handicap -- if Wizards could just get on planes reliably, the whole plot of Summer Knight comes apart.
Bu the world now is significantly different from the world of 2000 (when the first book came out). So much stuff is online now and even things like newspapers are dying out. It struck me how Harry had to use a pay phone in Storm Front. Now they're non-existent which for the series would make it so much harder for Harry to update/get in touch with others. Like the only way for Harry to make calls is from his house or someone else's (or to do so magically). This ends up cutting him off from his supporting cast to such a significant degree that it seems like it'll cause issues for the stories. Honestly, this doesn't seem to add anything for the story but does cause issues. Plus other series in this vein let their wizards use tech and it doesn't seem to cut off any story potential.
Isolating Harry is
the point. Causing issues is
the point. He's the hero, which means he has obstacles in his way. Him not being able to just whip out his smart phone to look something up or call an Uber or wikipedia his way to a monster's weakness is another thing that keeps the story interesting and shows how he has to really build his knowledge base.
Life
isn't supposed to be easy for Harry in the books. If he could just solve all his problems with a fireball or a smartphone, that just plain is not as interesting to read.
And, going back to the previous point, Harry Dresden's stories are
massively influenced by old-school noir detective fiction from the 30s-50s. The limits on technology are an enforcement of that, so that Harry has to go through the same sorts of leg work that old PIs would have.
