It's not necessary to use 'have' in speech. The perfect tense is already implied through your use of 'seen,' the perfect participle of the verb to see, so there is no need for the auxiliary verb to have.
It is very unelegant in writing to omit the 'have', though. If you write with poor grammar it makes you look inarticulate and uneducated. Speaking is fine, speak however you want as long as the message gets across.
The reason there is a difference between written language and spoken language is that most communication is through non-verbal cues like tone, speed, facial expressions and body language. None of that can be expressed in writing. So when you write you must be as precise as possible, so that there is no room for someone misinterpreting you. When you speak on the other hand it's ok to be somewhat incoherent with your words, since your non-verbal cues do most of the communicating anyway.
Context is also important for speech patterns though. You don't want to be too liberal with not following the rules of grammar when you speak to people in certain circumstances.
In a professional environment or anywhere else where it's important for your listeners to have a good impression of you, you should follow at least some rules of grammar. It's ok to use colloquialisms, but not ok to ignore all the rules of grammar.
On the other hand in a more casual setting, like speaking to your friends and family, or when you're a customer instead of a worker, it's fine to ignore all the rules of grammar if you really want to.
