Why repeated lumbar punctures to assess neuroprotective trials?

In response to one of the comments from my posting in relation to disease outcomes.
In MS damage to nerve processes called axons releases proteins into the spinal fluid. One class of these proteins are neurofilaments, which  supports the structure of the axon. I think of neurofilaments as the scaffolding or supporting structures of axons. What is important is that if neurofilament levels are raised in the spinal fluid it indicates that axons are being damaged at that point in time and is predictive of a worse prognosis. Effective disease-modifying and neuroprotective therapies should reduce or even normalise the levels of spinal fluid neurofilament levels. This is the basis of our proposal to test neuroprotective therapies. The study design is to take PwMS in the progressive phase of the disease and do a lumbar puncture to measure the level of spinal neurofilament levels. Patients are then randomised to a putative neuroprotective therapy or placebo. After 6 months another lumbar puncture is done and the spinal fluid neurofilament levels measured. If the neuroprotective therapy works it should reduce the level of neurofilament levels in the spinal fluid compared to baseline and this shoudl not occur in PwMS on placebo. This sort of trial means an answer can be done very quickly within 6-12 months on only 30 subjects. At the moment clinical trials in progressive MS involve hundreds of patients and last 3 or more years. 
What do you think? Would you have two lumbar punctures? I must remind you that we have gotten much better at doing lumbar punctures; we now use atraumatic needles and sonar or x-ray guidance that limit the potential complications.