People forget to notice one thing in Marshall's ODI statistics.
He most played ODIs in the 80s and his numbers in the 80s read 24.52 at the ER of 3.39. Only Garner and Hadlee had better ERs in that period. Marshall's and Holding's were about the same. Marshall did not take many wickets but he kept it really really tight. Perhaps, batsmen gave him respect for his reputation as a test bowler and were happy to just see off his overs.
Now let's check Waqar's ER in the 90s (his peak years). He is not even among the top 50 economical bowlers in that period. 4.61 ER was extremely poor for a bowler in the 90s.
Now, before Waqar's wicket taking ability is mentioned. Waqar's wickets/match ratio is less than 2. So, overall it does not make any difference if he took 1.6 wickets a match as compared to let's say Ambrose's 1.3 wickets per match when Ambrose gave 12 runs less in every match.
Also, continuing with Ambrose as an example, Ambrose took more quality wickets (ratings of batsmen) so he enjoyed far superior ODI bowler ranking than Waqar.
BTW, Marshall's highest ODI ranking was #1 and rating points were 891 as compared to Waqar's #2 and 778. Malcolm also spent more time above 750 mark than Waqar.
Here is the link for your reference:
http://www.relianceiccrankings.com/...ted=1833&name_selected2=0&name_selected2=1685