After the Detroit - Memphis - Toronto, Detroit has 4 point guards on the roster:
A trade of Nolan Smith and Luke Babbit salary-wise is almost an exact match for what Bynum makes (both are ~$3.2 million for 1 year)
Looking at 82games data, Bynum is basically an average offensive point guard and slightly above average defender. His career PER is right around 14-15 at 18 minutes per game. More effective than Nolan Smith or Ronnie Price, giving the Blazers the option to get Lillard a little more rest while not digging the team in a hole while he is on the bench. If it works out this year and you can resign him at $3M a year, you could have a decent bench PG for two or three years.
For Detroit, swapping one 4th string point guard for another isn't a big deal. Additionally you get chalupas, er, Luke Babbit. Since trading Prince, who was eating up 33 minutes a game at SF, there is a need for a body to plug into the SF slot. Sure, you can distribute thost 33 minutes between Jerebko, Villenueva and Maggette but Maggette is the only true SF of those three. Why not take Babbitt out for a test run?
Unless Detroit is planning on doing something with 1 of their 4 point guards ,it makes sense for both teams.
Also, you can swap Price for Smith or Claver/Pavlovic for Babbit and the salaries still work this year, but Price/Claver/Pavlovic are all on multi-year deals.