Of course knowing there's a time limit might increase the pressure and excitement a bit...
This in spades.
I've included Time Limit type elements on every map I've submitted thus-far. The biggest reason I add them is because when you're given an infinite amount of time to accomplish a task, then it is literally, only a matter of time until you finish. I don't personally see why a map should be deemed above "Medium" (or even "Easy" in some cases) if there isn't a mechanic that forces you to work faster than you might otherwise be comfortable. I like the chance of failure resulting from a too slowly executed plan.
Even if the map presents heavy creeper. But mass-spawning your attack units, backed by the packet limit of 32 packets per second, just doesn't strike me as being the epitome of challenge in this game.
... there are other ways to make a map increasingly difficult...
Of course there's always a turning point, but good maps makers will be able to postpone that moment as long as possible, sometimes until 1 minute before finishing the map. Randomness is not needed for that. You can for instance stack delayed emitters (so the amount of creeper increases over time), use walls to release more creeper, make the race for every tech a mini game, or limit the amount of units available by withholding the tech and only supply a few (initially locked) weapons (so losing them might cause failure), etcetera.
Granted. But what I've found, more frequently than not, is that there always comes a point, well beyond the onset of difficulty effects, where all the player needs to do is hunker down long enough so that he has enough energy and storage that when he finally advances, the only thing stopping him is the sudden onset of heart disease caused by the player spending too much time playing on the computer in the first place.
Seriously though, "Holding the Line" as waves and waves of creeper approach is only challenging (to me) so long as you can't keep using the same strategy forever. Steadily incrementing the creeper spawn rates is a nice choice-- One that I have used to some degree in the past-- But with how powerful some Turtling strategies can be, it just seems like there tend to be only two real options: Forcing the player to fight so aggressively that Attila the Hun would soil his pants at every mention of this player's aggression, or capping the time they have. Option 1 is a valid and real option that I have seen before..... In maps I can't beat. Which means that I can't make maps that use that strategy because, at least for the time being, I can't beat those maps, and as a result, can't submit maps I can't beat. If/when I become a better player, that will be a tangible option for me, but until then, Time Limits are how I roll.
And shy of all that, I would point at maps like my Limit Break
series, where taking as much time as you want means trivializing the fact that you're supposed to be given limited energy and storage for the duration of the map anyways.
When a player is less experienced and takes longer to finish a map than the average player, he's already 'punished' by getting a lower score, so why hurt him/her twice? When you are about to complete a map after a long battle and then get surprised by such a thing, would you start playing the same map again just for those last 5 minutes?
Well, I personally never set time limits above half an hour (my highest is 21 minutes on my recently posted Sequence
map, and that one really resets itself, partially, every 3 minutes anyways, and is, above all else, more a concept map) but I would draw attention to some of my play-testing for my earlier maps, specifically, Uphill Struggle II
and Limit Break II
. Both of these maps were in play-testing for weeks (when they really only took about half an hour each to construct in the first place) because at the time, I wasn't good enough to finish my own maps within the constraints I had set. But by practicing on them, I got better at the game and was later able to break the time limits by a substantially larger span of time than I had ever anticipated would be possible. (And then other players shattered MY record. Ego == Obliterated)
So just my two cents. I certainly understand why others might not favor it as a mechanic, but I thought I'd offer my own insight.
EDIT: Added links, since it occurred to me as an afterthought that it might be beneficial for reference.
One reason I can think of is that if forces the map classification in the maps view page.
shorter maps get more downloads. so if all the times on a map are short times, then the map gets downloaded more times. Just a thought, I doubt anyone put that much thought into it.
This has occurred to me, but only as an after-the-fact thought. I swear it.