Imagine you do regular builds on your project (e.g. nightly builds, continuous integration) then you would need to manually enter the different build names in the Jenkins job configuration (provided you want to use helpful TestLink features like the test result matrix).
If you don't do this, then all subsequent test runs' results will be recorded under the same build name in TestLink (provided the build is still "open")

What about this idea: If you leave the build name blank in the job configuration, have the plugin create a new build name each time the job is run in Jenkins.
Proposal for the name would be the date and timestamp, e.g. "build_20110518_090530" i.e. "build_yyymmdd_hhmmss"

This would allow for maimum flexibility. Either all test results are stored under the specified build name (e.g. "nightly build") or you have a new build created for each run of the Jenkins job.

Also a configuration of the build name via environment variables with wildcards could be nice.