It is always a bad thing. Because it means the code is sloppy, even if the vulnerability is not directly exploitable but you never know, an experienced hacker could find a flaw that you just cannot see at the moment. Or even a smarter user who thinks outside the box could find something that you missed.
So you should not assume that is harmless.
In fact you could have posted the vulnerable code in question, that would have made the discussion even more interesting.
The fact remains that the application is poorly coded and buggy. It if is buggy then it is not reliable: there are probably many logic and programming flaws in it, and you could imagine that it sometimes behaves incorrectly or returns wrong data.
You are not giving any details about that front-end but I am wondering what's the point of it. Isn't there a commercial or open source front-end application on the market, that you could use instead ? Then you wouldn't have to worry about this problem, the only thing you should do is set the respective users rights in the database and tighten access.
It's not the front-end that should enforce access rights on the database. Security has to take place at the source. In fact the front-end can probably be bypassed. Since
postgresql is mentioned in the tags, there are quite a few GUI tools for it, not to mention command line utilities. What prevents people from using one of those tools to connect to your database ?