However I would also argue it provides a huge additional benefit of providing cover so it is easier to for a incumbent to stay in power...
Lets say you have a mayor race someplace where about 60% of the town wants to get rid of the mayor lets say they were not happy about a referendum, change in traffic patterns, garbage pick-up etc. However two other candidates run for office besides the sitting mayor, so the sitting mayor gets 40%, candidate one gets 35% and candidate two gets 25%. Or you even have 4 candidates, the sitting mayor ends up with a huge advantage. If he goes head to head with someone he is going to lose, but with enough other candidates, he really can't lose.
Even using aldermanic elections, I don't know if the law applies to the city of Chicago, but imagine how much harder it would make getting rid of a sitting alderman in the city.
There is one case where it could actually make life a lot harder for a sitting incumbent, that is within an aldermanic race in Aurora. Lets say you want to defeat an alderman because you didn't like how he voted/acted in the whole clinic thing. You get 4 candidates in the race, but make one the obvious anti-clinic candidate and have the anti-clinic forces work hard for that candidate. You then also work to just increase the sitting alderman's negatives some, that will make some folks stay home and some folks vote for the other candidate. You also get the other candidates to be single issue on other hot button issues. Lets say traffic and crime, so they start to pull votes of the incumbent as well. Since an aldermanic race can really be impacted by turnout or the lack thereof you can get turnout up for the other candidates and down for the alderman. All you need is one of your candidates to beat him and you win. You couldn't have beat him on a single issue but with enough candidates you could.
Regardless it changes the math.