Well, I'm not sure if I agree, I guess it depends a lot on who you are and where you are at in your life...I know I discovered a lot about myself in 3 1/2 months, and it didn't feel like much of a holiday toward the end!
But I wanted to add a practical consideration, in connection with the initial question about differences between short and lengthy: I think you have to plan short trips out a lot more - seems contradictory, doesn't it? But if you're planning to go someplace for a week or two, because that's how long you have (don't flame me, we have job/school to take into account), you have to really think about what you want to do, when things are open, how long it takes to get from place to place, etc. If you take a longer trip, you have the flexibility to change your plans, take advantage of good weather to be outside (or opt for an indoor day when it's pouring out), take detours, etc. We are currently planning a week-long trip to Italy at Easter, and I've just spent two days working out driving times, where to stop and eat en route, where to sleep en route, and figuring out what we can see in the limited time available.
Obviously, if it costs you $2,000 just to get to another country (which it very nearly did when I went on my ... middle-sized trip) you don't want to go for only a week. But if you happen to be in a position where you can drive to another country, or fly there for $30, isn't it better to go for a short trip, rather than none at all?
There are a lot of trade offs when you are dealing with a short time period, but I still believe a short trip is worthwhile if it's all you can manage. Of course you don't get that open-road, freedom feeling, but part of the reason to travel is to open your mind to other cultures and places, and I think you can still do that when time's limited.