Depending on the airline you may not know how long you have between INDOC and training until you are actually at INDOC. I was an ART, got hired at my airline, resigned my ART job (because it was the worst job I ever had so I was more than ready to go), and went to INDOC. While there I found out I had 7 days between INDOC and Day 1 of training. During those 7 days I was responsible for getting through the at home training portion of the course and ready to take the written test Day 2. No way I would've wanted to deal with working the ART job or even out-processing from it. Much much much more important things to focus on: airline stuff and family stuff.
Obviously the flip side of it is if you have 6-9 months off...personally I wasn't willing to take a chance of having limited time to do anything more than study. If you are a reservist you could probably do the trough thing just as easily as do the ART job IMHO.