Purpose of Time Limt:

-People won't get all the gear or whatever lies in there in a day or 2
-Encourages more playtime runs within those Dungeons
-Gives reason to keep going to that Dungeon to get that gear you always wanted thus making it more Rewarding

Without a Time Limit:

-People will bum rush the Dungeons till they completed everything in there within a day or 2
-When they finish they will complain that they have everything and want SE to pump out more Content now
-Less initiative to want to do Dungeons because it will get old without having a need to go there
-The items you get won't be nearly as rewarding if you just rush everything

Also Reguarding Grand Companies:

-These Dungeons are in Relation to the Grand Companies please do make note of that as to why there's a time limit
-In, the future whenever Grand Companies come out (probably this Summer) That 60 min time Limit may just be there for that reason too, for activities within the Dungeon itself.