For a mereological approach with a bottom element see Link (1983). For reasons why the bottom element is not desirable see Link (1998:284ff). He discusses this at length when talking about processes. Specifically: "[...]processes are mereologically structured entities that form my basic
particulars. I assume that their mereology is classical, but not explicitly
atomic. In particular, there is no such thing as "the null process," i.e., the process lattice doesn't have a bottom element.". This is not very specific, but I would suggest you read that subsection to get a feeling for the philosophical arguments against a bottom element.
There are other reasons for not wanting a bottom, which I prefer. A simple one is, when dealing with spatial entities (e.g. point in space linked to events, as most event structures assume them nowadays), you would not want to assume there to be something that is a part of everything (which bottom is), committing us to an overlap across the domain -- which is implausible. This is my own reason and also the one given in (Geach 1949, Simons 1987, Lewis 1991 and Varzi 2007).
So the reason is more why you'd want a bottom, rather than why you would not want one. Technically we don't lose much without it, as we still get structure isomorphic to a complete Boolean
algebra with the zero element removed (as shown in Tarski 1935).