There are some advantages over a netbook- Instant on, size/weight, battery life, affordability. Netbooks running full OS tend to be slightly underpowered. Although they do have the advantage of a full OS and all the software that comes along with it, they typically do not have GPS which some Android tablets might.
I had tablet PC's in their early days, smartphones since before they were called smartphones, and netbooks since they came out. I use a high end laptop and don't even own a desktop anymore. I dont like carrying or opening my laptop in a meeting and the battery wont go a full day at a conference. My netbook is nice for this as it is much less obtrusive and has a better battery life. It is still a bit obtrusive to open in a meeting and it is another full blown PC to maintain. A tablet (Android, iPad, whatever) should be able to sync whatever docs I need on the run or in a meeting, keep email and calendar all synced up, easily take short notes, have entertainment availability (news, sports, video, etc). Ideally, it would even be capable of running a presentation (PPT or otherwise connected to projector).
Sure, most of that can be done on my phone too. The bigger screen and keyboard are nicer though. It really does depend on the personal user and what their preference is. As a secondary device for many a tablet would work well- for others a netbook- and for some their really is no alternative to a full blown laptop.