Never worked ML, but in other TWRs .... there's no point lining someone up if you think you aren't going to get departure instructions (APP maxed out) so it's better to leave the rwy available for the next arrival rather than cause a go-round.
Having said that, I was trained in the "move metal shagga" school of ATC, so a big fan of the "behind, behind". My pet hate is pilots confirming they are "ready immediate" and then stopping on the runway for PDCs.
If you can't do a rolling take off, then let TWR know and don't accept "ready immediate" .... please.
