Honestly, it probably causes more trouble than it's worth. A LOT of people have problems with the concept of what transferring an upgrade actually means for the contract status of their line, that's why it can be difficult how to explain using a dumbphone line to get a subsidized handset for your unlimited data line. I'm sure they get plenty of calls demanding to know why there is an ETF on Grandma's line that has had the same phone for years, only to realize that all those upgrades they transferred actually did something...
It also give them another chance to verbally push the new plans.
