At least one study I've read indicated that megadoses of vitamins may be useful, by supplying an excess of vitamins to parts of the body which do not always get an adequate supply, like the bladder (which was the focus of that study).
I don't believe it is wise to simply dismiss the possibility that megadoses could be helpful based upon some negative studies. There are also positive studies which do indicate that some supplements are effective when megadosed.
Rationally, we should examine which supplements might be useful in megadoses and which might be harmful, then megadose those which are of positive value and keep the rest at their recommended levels.
One could argue that the body has the capacity to handle many different levels of supplementation, since natural foods have wildly varying amounts of supplement content.
Searches on Google Scholar and PubMed reveal quite a bit of research regarding vitamin megadosing, with many positive studies favoring megadosing for specific conditions.
For example, a meta-analysis on vitamin C megadosing indicates that 0.25-1 gm/day lowers the incidence of the common cold by about 50% in people who are under short-term physical stress. Another meta-analysis indicated that the duration of the common cold could be reduced between 8%-18% depending upon age, in doses of 1-4 gm/day of vitamin C.
There are also many factors which should be considered which may skew the negative findings for megadosing supplements. For example, athletes utilize more vitamins than do sedentary persons. Studies which find that megadoses are harmful or have no benefits in the average population might very well discover that they have considerable benefit in athletes.
The problem with many studies is that they can only provide information for the specific cases they are testing, including the dose, the frequency of the dose, the specific population, the length of the study, etc. It's difficult to account for everything in any research, but when it comes to megadosing, there is a relative deficit of studies, and the studies which do exist tend to favor only a few of the vitamins (A, C, D, and E).
So don't write off megadosing too quickly...but also don't accept it too easily. There's still much to be learned and some evidence that some megadosing is useful.