The LGPL is not always compatible with commercial licenses.
"In order to combine two programs (or substantial parts of them) into a larger work, you need to have permission to use both programs in this way. If the two programs' licenses permit this, they are compatible. If there is no way to satisfy both licenses at once, they are incompatible."
It is a common misconception that you can always combine LGPL code with code under any other licenses without a second thought. The only time LGPL code can be included in a product under any license (with minor restrictions) is if the resulting work is one that only "uses" the LGPL code and is not "based on" the LGPL. The safest way of drawing the line between these two concepts is to treat any work that includes modified LGPL code as "work based on" LGPL code. In such a case, the LGPL is "viral" just like the GPL, meaning the resulting work must be released under the LGPL as well. See clause #2 of the http://teem.sourceforge.net/lgpl.htmlLGPL.
Remember that the LGPL was originally intended for libraries (hence the name Library GPL). Libraries are usually written in such a way that it can be plugged into any larger work without modifying any code.
IMHO, there are few cases when Tiki's source code can be used unmodified in any outside project as Tiki is large and messy, lacking any good code documentation or API's. Therefore, there are probably few cases where Tiki's code can be used while avoiding the viral nature of the LGPL.
From this, I've come to my opinion that Tiki doesn't put the crucial part of the LGPL to good use; in other words, there would be no great difference in Tiki code usage if the license were changed to the GPL. Relicensing would, however, open up many possibilities as there are more GPL web applications than LGPL/BSD ones.
P.S. I am unsure about clause 2a of the LGPL. "The modified work must itself be a software library." However, since the LGPL can cover works that are more then mere libraries, I believe this term no longer applies.