Spare input values on elements - good or bad idea?

Summary:

After an element has been processed, no new input values can be added. Is adding spare input values just in case a viable solution?

Content (required):

I have seen some people create additional input values on elements, with display disabled, as spares so that if an additional value is needed after it has been used, you can bring one into life.

On one hand this looks a bit untidy, but it obviously serves a useful purpose because the alternative is rebuilding a whole new element and dealing with all the dependencies that come with that.

Is this a reasonable solution? Are there reasons not to do this?