This is not a sensible idea at all. Not just because of the access times, however, but due to the way that many tape drives write to the tape.
Each write operation to a tape moves the End Of Data mark to the position on tape where the last write operation finished. This prevents the drive from reading beyond the end of useful data. Now, if someone were to try to use a tape drive for random writes, the End Of Data mark will be in the wrong place, preventing access to the rest of the data lying beyond that point.
Tapes do still have their uses in certain organisations, but trying to use tapes as large disks is pointless from a technical point of view, unless you are prepared to restore the entire contents to disk cache in order to edit a file, and then re-write the new tape contents back down to tape in linear fashion.