Zimbabwe simply does not have enough quality to afford not to go into ODI without four quality frontline bowlers. Sides like ENG(Woakes, Ali) and SA(Morris, Parnell, Phehlukwayo, Pretorius) can do that.
There was a time when Zimbabwe used to get that point. What happened? Utseya, Price, Cremer plus a seamer (e.g. Mpofu) used to be Zimbabwe's best approach. Everyone understood that you don't mess with International cricket. Zim were doing that whole 4+1 which Streak vowed to break-up in his fast bowler search

(except he ended up killing off slow guys like Mupariwa who were delivering - AUS kep Bracken, ans SA kept Rusty Theron

).
Yes Elton(who can bat extremely well), did bowl, but as everyone has seen, his stats were very much up there at the time. Even when risks were taken with guys like Greg Lamb in the attack, he was doing well against full strong sides in the CWC, as well as bilaterals like WI, IND, SL and SA - not flopping against Afghanistan and weak SL and WI sides.
The point is, as much as Mire, Hami, Chamu, Williams, Raza and Malcolm are not good enough to be one of the 4 bowlers in an ODI attack, only good enough to make up the 5th bowler, same applies to Nathan Waller. Except Nathan Waller will NEVER be good enough to bat in the top six for Zimbabwe ahead of Malcolm

, Hami, Vusi, Craig, Elton, Mire, Sean, Raza, Chamu, Taylor(or even Mutizwa, Musakanda and ), neither will he be good enough toovertake a lot of the seamers (Chatara, Panyangara, Shingi, Jarvis) and spinner(Cremer, Wellington, Chisoro), particularly with Zimbabwe trying to develop the likes of Ngarava and Mumba.
That's why Nathan Waller needs to choose, and hopefully develop enough to be considered ahead of Malcolm Waller for example. Cartwright did that. Steve Smith did that. Even Afridi did that - in the end he simply said I'm a bowler and outbowled the competition! Shoaib Malik failed to do the same and he was very often left out of the XI. Maxwell is going through the same.
In the end I think Nathan Waller will suffer just like Mitchell Marsh - not good enough for either role, which is made worse by the absence of a good keeper-batsman.
These kinds of players only succeed in their selection quests if the team has a very good keeper-batsman.