Real sex is more fun if a person knows exactly how to get her or himself off. How can you help your partner push your buttons in the proper fashion if you don't have any idea yourself? Everyone is different, after all, and we don't come with an operating manual. So, it's good for her to experiment in such a safe manner. Secondly, I'm sure she realizes that men don't vibrate without vibrating cock rings. And overuse of the vibrator can make it more difficult for her to orgasm without it. But, the same is also true of boys who use a strong hand while doing the deed. But, some play won't harm her. Thirdly, you make it sound like a woman who requires a vibrator will hop off and finish things off herself without any participation from her partner. But, there are so many different things a couple can do, for instance, he can wield the vibrator, that don't make it a problem. So long as both people are satisfied with the type and frequency of the sex they are having, I fail to see why including a toy is a problem.