In my honest opinion Unlimted sets the tone for Alternative for a few reasons to be honest. For one if you dont play unlimited then you have no clue what Alternative V is, why Kasumi is so quiet and attached to the brain; along with various other plot points so by saying you can read a summary of it you will still be in the dark for a good portion of some of the things mentioned and talked about. Unlimited is crucical in my opinion as it sets the stage and tone and while Alternative goes into detail about the entire plot and starts closing some of the plot holes that unlimited laid out on the table. and to say to skip it would be like telling someone to go read F/HF with out having read the other 2 routes and expect people to know what is going on. In closing though if you skip unlimited then you dont really feel the sense of urgency as to why Takeru does some of the things he does in alternative on top of that you really cant connect with him when it comes to various character deaths and why he takes them as he does.