Should a host always be ready to cater to surprise plus-ones? 

Hosting an event for friends and family can be a big responsibility. Preparing to entertain requires you to think about a lot of things at once, and even then, there is every chance that when the date rolls around, you will be thrown a curveball that you couldn’t have anticipated. You have to balance being organized with maintaining a certain level of flexibility.

At the same time, being a good guest is just as important as being a good host. If somebody has gone to the trouble of inviting you to an event that they have put a lot of effort into, the best thing you can do is be polite and respectful of their expectations. Fail to do that, and there is every chance that you will not be invited again. 

While it is true that some people who organize parties and events for their friends can be unreasonable when it comes to what they expect from their behavior, it is usually not that hard to be a decent guest. So long as you do not go out of your way to cause extra difficulties for them, you are probably in the clear.