If you do a user-centered approach to design, should you allow others to try your game? The Answer: YES!!! Even if you are designing for a muse in particular, attempting to test on a user level allows for more flaws and quirks to be seen than through one person and besides that, one could see if the game is fun for more people than just you, and that is not just important, that is vital! Having the game be accessible and seem fun by many people is what makes the difference between a good game and a bad game. Make your game friendly. Make your game polished and inviting. Have players get sucked it and have the theme elements flurry them so that to daze them.
Besides that, what else could one learn through having external playtesting? Well, for starters, one could get fresh input. Your muse may know what you are trying to achieve or may have the experience of trying the game earlier in production and then getting accustomed to it, or in other words, start loving it. This could make her less careful about what the game is supposed to transmit in general and it could make her not notice certain bugs or elements that are supposed to be different.
External playtesting is also good for PR purposes as they can not only provide good feedback, but they could be used to create hype around your product which in turn could lead to a good turnout in sales, something that if you want to sell your game, you should do.