Much easier to influence our neighbours from inside their club. Much harder from outside.
Leaving gives a green light to facists / ultra-nationalists to start campaigning against all foreigners and immigration — and we know where that can lead
We leave certain treaties that give workers rights; stop doctors working ridiculous and unsafe hours; generally what I would call civilised laws
Scotland will almost certainly vote to leave the UK if there is a vote to leave the EU.
Economic
Stepping away from a bunch of existing trade deals will cause an economic shock which will cost jobs and cause business failures. This actually looks pretty certain.
View from outside countries/economies may cause pound to fall / currency uncertainty
Why Brexit is a bad idea:
It won’t solve the immigration question. Govt and economy still needs migrants to grow and for health etc to function, and still need to face what to do. There are really no shortage of levers to control immigration now; there is just a fundamental conflict between economic prosperity and lowering immigration.
It will lead to currency uncertainty
It will lead to new and difficult trade negotiations