Hi, i have been developed a lot of small php websites over the last year using Smarty.

I am now about to start working on a big project for myself and i am wondering if i actually should be using a template engine or not. Can you help me with the following questions

1 - is the use of template engines seen as bad practice and unprofessional
2 - do facebook, twitter, myspace etc use template engines
3 - what is your opinion on smarty
4 - what are the disadvantages of using template engines and or smarty

Thanks in advance for your help...