Drug resistance in microbes varies with the drug, its origin and dose level applied. Therapies with a single agents are only found successful when you are providing dose first time to treat any microbial disease, the best example is use of quinine against malaria, but later on Plasmodium has developed resistance and physicians have increased the dose level that was considerable 2.5 times more than the normal physiological dose, than a combination therapy was applied to treat the patients, and physicians noted good success rate, but just after 4-5 years again a second line of resistance was seen in malaria pathogen and similar happening in other drug resistant pathogens. In response to decreased susceptibility the reason was established as new mutations acquired by pathogens in genes mainly which synthesize catalytic scissors. But to eradicate disease and to stop resistance a physician should change the drug combination, if drug will not remain present to the microbe to read its formulae, may stop genetic machinery to synthesize most required enzymes by the microbes. I agrre with your opinion and view on combination therapy with two or more antibiotics is superior to monotherapy. It is much better in achieving clinical and microbiological success to check rising resistance in infectious agents.

The topic of monotherapy vs. combination therapy is debated quite a lot, and proponents of both sides do cite studies that claim to have evidence for their respective stance. Studies are notoriously difficult to plan, and probably even more difficult to implement for several reasons. I don't think there's any general rule, but rather each case needs to be assessed individually based on infection, bacterial strain, available drugs and probability to attain the PKPD target. Basically, the more complicating factors there are, the stronger the case for combination therapy gets.

With regard to selection of drugs to combine, it's a jungle out there. My limited experience is that there is considerable strain dependency, so that a combination that works for one MDR-PA doesn't necessariy work for the next. We also have difficulties in vitro with testing of combinations that are clinically relevant, endpoints (synergy, increased effect, antagonism, resistance selection) and how one should administer the combination (all at the same time, with some time between the drugs or sequentially).

I hope this helps you to see that there is no simple answer to your question for now. But looking at the rates of resistance, the slow (or nonexistent) discovery of novel antimicrobials and history of eg. tuberculosis, HIV or even cancer therapy ending up with combinations, I sincerely believe that combination therapies will become increasingly necessary.

If you go on reading the published article on in vitro synergy of combination therapy most of find a synergy when antibiotic are tested in combination.However there is no many report on clinical studies. You can find a article below

Simsek F, H Gedik, MT Yildirmak, NE Iriset al. Colistin against colistin-only-susceptible Acinetobacter baumannii related infections: Monotherapy or combination therapy? Indian. J. Med. Microbiol. 2012; 30, 448–452.

This reports contrast in invitro and clinical outcome. Combination therpay in case if you inhibitor or chelator such as clvulanic acid or tazobactam/sulbatam etc they are going to be effective as they inhibit or for ESBL/MBL but combination of antibiotics as for example colistin plus merpenem or other are still to be in investigated in field.

Multiple different mechanism of action of antimicrobials on  microbes- help us get rid of those pathogens, so that there is no re-infection (reactivation) with such multidrug resistant difficult to treat bugs. 

Very difficult answer. As Dr. James John says, there is not correlation between in vitro vs in vivo results. You may support on clinical basis (infectious picture) by using one or more antibiotics, and the results of the antibiotic resistance pattern.

Treaating infections caused by multidrug-resistant Gram-negative bacteria is challenging due to limited options for  therapy. Combination therapy has garnered great interest recently, with the goals of ensuring appropriate therapy with at least one active agent, and achieving synergistic activity among the anti-microbials used.  Most of  the data are  derived from in vitro synergy studies.But the  correlation between in vitro vs in vivo studies are contradicting.In vitro studies suggest an advantage of combination therapy over monotherapy but clinical data are scarce, and are comprised of retrospective and a few prospective observational studies. Well-designed randomized trials are needed to better elucidate the efficacy of the various combination regimens. 

Combination antibiotic therapy has synergistic activity against MDR organisms. But clinical data are very less and still to be in investigated in field.

Yes, combination antibiotics therapy has synergistic activity against MDR organisms.we are doing research projects on it also.