Personally for me, it comes down to how much bulk you can deal with. Any wallet case is going to double the size of your iPhone.
Yes, you'll be able to not have to carry the bulk of your wallet, but part of that bulk is transferred to your iPhone, which you have to remember is being used on constantly on a daily basis, compared to a wallet that sits in your pocket. Constant usage of your iPhone and having to deal with that extra bulk may not be favourable for many, especially if it's a flip wallet case, where the flap can constantly get in the way.
This is not to say it's terrible, it's just not suited for everyone. Personally, if you do decide to go with a wallet case, I'd recommend the Twelve South BookBook. It has a detachable slim case from the flip wallet portion, which means you don't need to deal with the bulk all the time!
Shameless plug, I've got a review of it
on my channel if you're interested in learning more about it.