OOOHHHH I know!!...

The tap water filter is AWESOME and will turn out water just as pure as ro/di.... I used to keep discus and fw planted tanks and it worked AWESOME!!
why don't we all use it then??
here is the problem... lets say for arguement that the tap water has 300 ppm tds. now those with rodi will have this happen... the RO will take the water down to like 5-25 ppm TDS and then the DI will take that down to like 0-5 ppm tds...
all of our rodi units would take the tap down to 0-5 ppm tds if we just ran the water through the DI but guess what... we would have to replace the DI (the beads) frequently... some of us (with bigger tanks) would have to replace them after each water change. it is because the DI portion can become spent as you should know the "beads" change color.
thus, in the long run (and for some of us in the very short run) it is much more cost effective to first run the water through an RO membrane and then through the DI beads... the Di for rodi is kind of the finishing touch. Those that use RODI still need to replace the DI component periodically (annually at least we are supposed to). There is a way to recharge the beads but it is very messy and requires strong acids.
Bottom line: For small tanks the tap water purifier is great! but in the long run it will cost you more.
Also FWIW I have used RODI, I have used the TW purifier, and actually *cringe* I have been using tap water for the last year. My tank looks fine and the acros grow nicely. Remember that with our water if you have 300ppm tds... that 250 of that or maybe even 295 of that will be calcium e.g. we have hard water here. I only had to deal with a small diatom issue at setup, but once the coralline kicked in I have had no problems with algae at all. the moral of the story..... PRAY to the reef gods!!
