I used to believe that speaking properly was imperative. I made it a goal to use proper verb moods, eliminate passive voicing, and speak eloquently.
Then I realized one very important matter - Language is used to disseminate information. The least number of words used to communicate the most amount of information is the most effective.
The English language used to be vastly more complex. However, due to colonisation and the subsequent Anglicisation of various British colonial properties, English began to simplify. It simplified because people from diverse backgrounds and origins needed a common way to communicate. Verb moods and tenses like dative, subjunctive and genetive began to die off. Now, we have a simple language that can be understood and spoken by a third of the world, and that is more important than holding on to arbitrary rules and traditions. Comparatively, Finnish ,Swedish and Norwegian are so frustratingly difficult because they've never really been used widely as colonial languages. Other colonial languages like French, Spanish, and Chinese have all seen simplification.
So, if we start using "Seen" without the present perfect helper "have", and it indicates something that happened in the past (and the person listening to this understands it) then it's one abbreviated word with the same meaning conveyed.