Why is that? Because your membership is almost for free if you look closely at it. You get a free Linden Home or a 512 sqm for free as a member. The land above is 512 sqm. Ok not many prims but there is a lot of low prim stuff in SL so you can fill your parcle anyhoo =)
If you dont want your plot you can donate it. Some landlords give you a free rental if you donate land. Ravenglass rentals for exampel.
As a Premium you also get 300 L$ every week. 260 L$ is 1 US $ so its good money back! You only pay a few dollars a month (depending on VAT) and you get almost all back this way. Good deal!
I myself has had a lot of awesome help from the Live Chat you can access as a Premium member. When i screw up I log in and get help instantly! If you are griefed you get help faster too.
The Premium sandboxes are wonderful to build in. No one disturbs you. You can build for hours in peace.
You also get some Premium destinations and gifts. Ok some ppl complains about the gifts but then some ppl have a need to complain about everything im afraid :O)
So my advice is, sign up as a premium member, its well worth the money. LINK: Premium membership.