This type of thinking is a double-edged sword. It's great to have a deadline as it will keep you working hard toward finishing the film, especially when you don't have a distributor breathing down your neck to finish. It's also great to incentivize your cast, crew and vendors to work quickly as they would like to see their efforts on the big screen at a famous festival too.
However, there are real downsides toward making a festival submission deadline the completion date for your film. You may end up compromising the quality of your film by racing through important steps in order to finish, or you may turn in a version of your film before it's really ready to be considered by a programmer. In both scenarios, you are not putting the film and its needs first. And it will be apparent to those who are assessing your film for their festival.
It is true that most festivals accept rough cuts but filmmakers should look at this as a viable option if the film is very close to completion. If you know your final version is not going to vary much from the rough cut then you are probably fine submitting it. Perhaps you are only finishing up the color correction or minor visual effects. In many cases, you are fine submitting before a final sound mix is completed but only if your production sound is decent. If the sound is horrendous and causing major wincing throughout then you are best not submitting your film before a sound mix occurs. Sometimes, festivals will allow you to swap out a better, more complete version of the film within 2 to 4 weeks of submitting it but don't count on it. It's up to the programmer and he or she may say no.
Bottomline, if you are finding yourself having to seriously compromise the quality of your film for a festival deadline then it's best to forego the submission to that particular festival. There are top-tiered festivals that occur throughout the year. It's best to preserve the quality of the film and your reputation as a filmmaker than to race to try to make a festival deadline.