well, I use these things almost every day. I've always have at leasst 2 'combi drills' on the go, (I renovate houses for a living). I have a Hitatchi combi drill, (drill driver,and hammer drill) , also a Hilti MF ( Moth.. Fu...) 24v big bustard thing. But last year I had to build a large area of decking, o treated myself to an impact driver. never owned one before, its like a lightweight stumpy drill driver. I researched well, and bould a Hitatchi. with 2 batteries. WOW, this little ting is amazing. I use it all the time. let me explain
a) a hammer drill action, effectively hits the screw bit into the wall with every bang, tus ending up ripping out the x screw head. Put in high torque mode without hammer, and it still rips out the x in the cheeesy headed screw, no matter how hard you push. BUT
b) the little impact driver is not overly powerful, but on ever bang, it hits the screw head rotationally , thus tapping it around that increases the ability to screw into any thing with ease. Plus they do not have a chuck, but a pull back collar, and the biits have hex shanks, so you can get 6mm,8mm.10mm, 13mm, and so socket extensions for coach bolts. I was amazed with the ease at which this little thing was able to drive 90mm coach bolts into damp joists, all day long. How I never got one before I'll never knw.
So I would say YES, you need both, but get a kit , a pair, then they will share 2 battereies, and 1 charger. Hitatchi are good imho., and I've destroyed the chuck head bearing in 2 De Walt drill drivers.