If ATC tells you to speed up and going .02 mach faster causes you to not have enough gas to make your alternate I'd blame your dispatcher for giving you almost zero contingency fuel.
Going faster in and of itself is normally not going to be an issue. It becomes an issue, however, when other factors are thrown in. Flying at a different altitude than planned. Holding. Deviating for weather. Some factors are put into the fuel planning, such as holding fuel. Often there is extra fuel as well to cover the deviating. The problem arises when a crew uses this contingency fuel while also flying faster than planed.
While a .02 mach difference between planned and flown is not that significant on a 1 hour flight, the longer the flight, the larger the difference between planned and flown mach, plus other differences (again, such as altitude, deviations), the greater the difference in fuel burn.
As an example, during the Chicago TRACON shutdown I had several flights where we were kept at a low altitude (14,000'), for over 15 minutes and our route of flight went around the Chicago area. Initially this was not planned and we did not learn about it until we took off. Flying CI we made the flight without an issue, though I had to keep an eye on the fuel. If we flew cruise power instead I would have had to deviate.
As for blaming the dispatcher that is bunk. The dispatch is a dual responsibility document between the captain and the dispatcher. If there is not enough fuel in the dispatch then the captain should never depart.
This argument is part of what I find infuriating about this website. A poster complained that pilots were "sandbagging" by flying too slow. I pointed out that many (probably most), airlines are moving to CI and SCI flight planning. A few immature pilots throw hissy fits and call me a liar for pointing out that crews have run into fuel issues by not flying their plan. I know this will come to a shock for many of the young pilots here who went straight to the airlines, but flight planning 101 that is taught to private pilots- If you don't fly the plan, your numbers won't work. Usually this is not an issue, but it can be.
Again to the OP. Sorry for the hijack. I'm not going to stand by while pilots who obviously have no clue about flight planning 101 jump all over me.