IMO, A short CAN devalue a feature, but it certainly does not have to.
Largely it depends upon how the three following aspects relate to one another: the planned feature, the short, and what your intentions are.
Alright, we all know shorts don't make any money (directly), they may lead to grater opportunities, but largely any potential revenue (not profit) is going to come from a feature.
If the feature is essentially a one gag/trick flick then... there's really not much you're going to have to put into a short that's going to A) have any relevance, or B) make much sense and pique interest towards the planned feature.
However, if the planned feature is chock full of interesting and unique aspects then developing a short or two that uses a single aspect of the planned feature's multiple aspects to cultivate interest or to test the waters then that should provide some benefit.
Think
Alive in Joburg leading to
District Nine, or even
Annoying Orange leading to a
half hour time slot at Cartoon Network.
The second aspect would be the short itself, or shorts.
As already stated, the short itself probably has nil chance of directly generating any revenue (not profit), but shorts do cost money to make, and it costs money to get them to be seen enough times to provoke sufficient interest in a feature length film - even the viral shorts need a little coaxing.
Now, if the feature is a one gag/trick flik then... what are you really going to put in your short that isn't going to ruin the planned feature?
The one clever twist the whole feature film depends upon... is gonna not make any sense to include or exclude from a short.
So, only make a short if you got a fairly decent arsenal of clever stuff to cram into a feature.
Finally, there's your plans, your intentions.
Do you really think you're going to make a one hit wonder that will skyrocket you to feature filmdom?
Do you really think if you make the short that overwhelming public demand will force a studio with means to throw money at you to please please FOR THE LOVE OF THE SHAREHOLDERS, WILL YOU PLEEEEEEEAAAAASE make the feature?
Do you really want to spend the time effort, energy and expense of peddling your short to enough audiences to cultivate interest in supporting the funding of a feature film?
Do you want to NOT cultivate an interest? Interesting gambit, there.
Understanding how all three of these work together as a codependent triangle should answer the question "do shorts devalue the feature?"
I myself do not.
I try to watch every teaser & trailer and read every spoiler & review, both public and critic as they both have different merits, before watching a feature length film - or not watch it.
To me if a show "is good" I'll watch it time and time again even though I already know the story and lines.
Some shows are worthy of a one-and-done viewing.
Others worth skipping.
If a feature film is good - watch it.
If it isn't - don't.
A short or teaser or review shouldn't change that perspective one iota.
