This will allow a bit more flexibility when implementing custom methods for other data sources.
Eg for dplyrXdf, I'd like the ability to specify whether an exact sample size is required, along with the output format (data frame vs Xdf file):
sample_n.RxXdfData <- function(tbl, size=1, replace, weight, .env, .exact=TRUE, .outFile=tbl_xdf())
{
...
}