Of course you get some bonus with LimeWire Pro, including the fact that you won't see the first dialog that ask you if you would like to contribute to the LimeWire developenet and support by giving a small fee with your PRO purchase.

But it's a fact that LimeWire Basic is fully functional, and it integrates almost all of what you'll get with PRO. So the payment is not to get the software but really to support its development.
There are other ways to contribute: you can buy some goodies too in the Limewire shop, or you can contribute to the open-source development and tests.

You don't buy really a new licence with PRO, because even the Basic licence is valid nd gives you the right to get the benefits of the Gnutella network with the most advanced technologies that are implemented in BOTH the Basic and Pro versions.

Just ask yourself what is really limiting in Limewire Basic, consider the utility you get from it, and your right to test and use it as long as you want, and upgrade it at any time. Now consider the costs involved in developing such free software, and it's legitimate that Limewire asks you, politely, to contribute if you like and use the software.

If nobody contributes, the software development will die, and nobody will get new free upgrades for their Basic version. So Everybody will loose a software that it liked and used so much, and nobody will see improvements that he would like to see.