I don't know, I think I have to disagree with most of you guys. You want to be a master of one service, but it doesn't mean you can't offer others. That being said, I wholly support having a tint guy, a PDR guy, audio guy or remote starters guy, who can come in and do cars while you are working too. It's money for you, money for them, and referrals will work both ways in the end as well. And who knows, you might have someone who wants their windows tinted and they end up being a customer for detailing as well. As long as you find guys who are good at what they do, I don't see a downside...
As far as going mobile, tinting needs to be done indoors. Otherwise, dust and particulate matter in the air will settle between the glass and tint and create noticeable imperfections.