It's definitely preferable to having two quality fours and no starting caliber fives. I suspect the big rotation will be as follows: Duncan 28 mpg, starting four 26 mpg, Bonner 22 mpg, Splitter 20 mpg. I didn't find Duncan's minutes to be an issue last season and whether they play him 25 mpg, 30 mpg, or somewhere in between, he's still going to progressively wear down in the playoffs. But in order to have their best possible chance of going on a deep run, they'll likely need a high seed and in order to obtain that, they'll likely need Duncan for more than 25 mpg.
I don't know that it's got to be a defense first player. What it's got to be is someone who's not a liability when it comes to mid range shooting, rebounding or defending. That's the problem with Landry, Millsap and Bass. None are good defenders, two aren't good rebounders and Millsap, despite his reputation as a glass eater, has not been a very good rebounder either the past two seasons. What I like about Johnson is that he's not a liability in any of those three areas and in addition to that, he's athletic and a shot blocker. This team hasn't had a big as athletic as him (in the rotation, at least) in a long time and they haven't a second shot blocker since the days of Mohammed and Nesterovic.
Anderson, though I don't see him as a fit on the Spurs (unless they get rid of Bonner), I do think he is available. Not because the Magic want to trade him, but because they need to acquire a significant player in order to have a chance to re-sign Howard. They don't have a lot of good, young, inexpensive talent, so if they're to make a trade, there's a good chance Anderson would have to be a part of it. I think that's why they traded for Harper during the draft.