Any reason not to add 'rowversion' column?
Other than the minor things you've already picked up on (additional storage requirements, and potential performance differences as a result of requiring more I/O for the same number of rows), no, I can't think of any real gotchas with adding a ROWVERSION column to these tables. 

One potential issue, though, with merge replication specifically, is if Access grabs a row from one of the servers, then tries to grab a copy of the same row from a different subscriber (not sure if that is possible in your architecture). The reason is that the ROWVERSION values are re-generated at each subscriber, so they won't match the source - this may make it look like the row has changed, when it hasn't. Maybe not possible, or maybe it's ok that such a row always be considered "dirty," but something to keep in mind (and probably test thoroughly).

You might also want to review the merge replication section in this document about schema changes in general.