I'm not offering legal advice in your particular case. In general I find it useful for whomever will be reviewing / writing the final version of the description to have a very clear idea of the invention and how it differs from the "prior art" (the predecessor inventions in an area). This helps both the drafter, counsel, and inventor arise at a clear understanding of what the invention is and isn't.
To start, it is perfectly acceptable to use a similar application as a general template, even down to sentence structure, so long as one clearly distinguishes the differences of the new invention. This isn't a problem at all (especially with a very early draft).
However, there is concern because one isn't going going to know the background and history of the particular application used as a template. (For example, was the predecessor invention granted? Did it have difficulties with the examiner? Was it drafted carefully enough that it actually is a good template? These are things one won't know.) Furthermore one won't know the weaknesses of a particular application, therefore it is always good advice to seek the guidance of a professional familiar with the art and knowledgeable about the various legal land-mines that lurk in an application so that they can potentially be avoided.
Another concern may be that the predecessor disclosed or anticipated some feature you relied on, or an examiner isn't able to distinguish the relevant differences between the old / new applications.
The worst case scenario is that one would complete a description, submit it to the patent office only to discover they included or left out something that hindered their case.
There are multiple concerns, but if carefully navigated with professional help, it's probably not a bad way to start.