Networking in iOS Swift
To use:

if optionalVariable == nil {} else {
    // do stuff with optionalVariable!
}


would absolutely not be recommended.

For starters, 

if optionalVariable != nil {
    // do stuff with optionalVariable!
}  


would be slightly better, but still, I can't even remotely recommend this.

Given that optionalVariable is nullable, what happens if it is made null inside this closure?

if optionalVariable != nil {
    doStuff(optionalVariable!)
    optionalVariable = funcThatCouldReturnNil()
    doStuff(optionalVariable!)
}


At the end of the day, we should simply minimize our use of forced unwraps.

if let variable = optionalVariable {
    doStuff(variable)
    variable = funcThatCouldReturnNil() // &lt;-- compile time error
}


If variable is not a nullable, then you can't assign the result of a function that could return nil to it without force unwrapping the result.

The exclamation point should mark to us that something special is happening here... and if we can use the exclamation point, we might need to rethink our method.

At the end of the day, we shouldn't be asking ourselves purely whether or not our results remain the same for both methods.  We should also ask ourselves whether one way or the other opens the path for future maintainers to do something stupid and write some bugs into your code.

If I'm assigning the result of a non-optional returning function to a non-optional variable, if it is later determined that the function needs to return an optional, then Swift and Xcode will politely alert me to every occurrence that I need to go look at for my variable.

And if I'm not using the if let syntax and instead using the ! force unwrap everywhere, then Xcode can't remind me, and I'm asking myself for a huge swampy headache of bugs...