Orthotics are heavy

research: do I need to use them?

It is generally accepted that if the knee can be made to track straight up and down during cycling, with little medial or lateral migration of the knee, then more efficient cycling mechanics will result. In my experience this can best be accomplished by placing orthoses and/or forefoot wedging into the shoe, adding frontal plane wedges to the cleats, adding shims to the cleats, and adjusting the bike fit. In addition, to other parameters that need to be assessed in the cycling evaluation, STJ axis deviation must be noted. STJ axis location has been experimentally shown to affect the knee joint moments that, in turn, affect medial and lateral knee movement during cycling (Ruby P, Hull ML, Kirby KA, Jenkins DW: The effect of lower-limb anatomy on knee loads during seated cycling. J Biomech, 25 (10): 1195-1207, 1992). I believe that medially deviated STJ axes often result in medial knee migration during the power stroke due to the STJ pronation moments generated by the pedal reaction force during this important part of the pedal cycle.
from http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=3315

or
Firstly it is important to note that an unnecessary and unwarranted orthotic will do nothing more than limit performance. Remember that, like you superlight superfast wheelset, cycling shoes and in turn orthotics are a rotating mass, and will slow you down. Therefore if there is no pending reason for their implication then perhaps you should be asking some questions.
Some of you may have had orthotics in the past, either prescribed by a podiatrist for your day to day footwear, or for sport shoes and more likely running footwear (or both). If this is the case, then please note that this doesn’t automatically mean you need an orthotic for your cycling shoes as well
from http://www.cyclingtipsblog.com/2009/04/orthotics-for-cycling/