It can affect suspension geometry, tyre wear, turn in and other things.
Giving the car a wider stance by increasing the offset will increase the load on the steering pump and can lead to premature failure. Apparently the 'bigfoot' look on Vitaras (eeugh!) and the like causes a lot of problems like this.
A few mm either way can't hurt though.
Another thing: when looking into tyre/wheel sizes I became less concerned about getting them perfectly matched, the difference in rolling radius between a new tyre and one due for replacement (or indeed different brands) can be as much as 20mm. The 'plus one' concept usually gives much less variation than this.
I hadn't thought about it before, but this 20mm wear on the tyres must affect the speedo reading somewhat. Hmmmm....