From the perspective of an information security researcher with plenty of time spent working on both sides of the issue, I can tell you that any attacker worth their salt will be persistent and use multiple attack vectors to get what they want. This holds true for bored teens, corporate spies and real penetration testers (not just dorks that get paid to run scanning tools on your network) alike.

I've spent the last decade doing mostly IT Security defense work. I don't worry when I see someone port-scan my servers. Those kiddos are almost always benign. I worry when I see that our IDS starts to correlate many subtle attacks over a long period of time from similar places (such as the same school, ISP, etc)