The concept of whether you should learn a flat serve first is similar in some ways to the question of whether you should learn the volley with the eastern grips first because they are easier.
As a high performance coach--yet one that also works with beginners, I ask the question: Should you avoid something because it is hard? Will avoiding it allow you to learn it easier later?
The answer to both these questions is no. While there may be a few exceptions, most players who learn flat first, especially if they are using the eastern grip first to serve flat, have great difficulty learning to serve with the continental grip and develop a mastery of spin serves. In addition to the continental grip, there are many differences in the swing path and body position, as well as a difference in the action of the ball itself as a result from spin that makes changing from flat to spin serves so difficult.
Also, since all top level flat serves still employ spin to a certain degree, learning to hit 'flat' as a truly flat serve will be limiting in consistency of a flat serve.
The time it takes a beginnner to gain a feel for the spin serve, the continental grip, and learn where to aim for it doesn't take much longer to learn than the overal timing, swing and aim of a flat serve (if the student has no other previous experience). However, it takes much longer to master the spin if a player has learned the flat first. And, heck, if a player has a bit more spin on his flat serve, then I don't consider that a terrible thing. Having no second serve (or one that is a puff ball dink) IS a terrible thing, in my opinion as it will nearly prevent any player from moving out of a 3.0 or 3.5 level in most cases.
Again, I would remind you that if you want to be a skilled player, I have a saying: If you avoid that which you want to achieve, you will only achieve that which you are trying to avoid.
Think about it. It has a lot of barring on players wanting to play more skilled tennis. Don't avoid something just because it is hard...or, more important, don't learn simple methods if they won't help you become a skilled player: If you do, you will only be a 'simple' player.