If you just make kit wines, a pH tester is a nice to have, as Masta pointed out; however, I have never had a fermentation issue with any kit wine, so I do not own a tester.
On the other hand, if you are going to make fruit wine, the pH tester will greatly improve the quality of your wine. As you probably know, balancing your must is the second best thing you can do to make a sound wine (the best thing is starting with the best fruit you can get!). When you balance your wine, you need to adjust the sugar, acid and pH. Most people ignore the pH and focus on the other 2. The only reason I don't carry one is just a lack of room in my store. If you ever come to the store, you will see what I mean.
Anyway, if you have the extra money and you want to make fruit wines, I would recommend you add a pH tester to your arsenal.