I don't know how this myth keeps getting propagated. It is absolutely not true, for both the GSM and UMTS systems.
You don't need to have a background in cellular engineering to understand that if you want to use a service in near real-time (i.e. SMS), it is going to have to consume resources then and now.
Your phone is not using control channels constantly. This is for good reason - the control channels are extremely limited in capacity, and using them frequently would consume your battery as well.
Your phone is only using control channels typically when moving between cells or locations areas. You can easily see this on GSM phones if you have an old radio nearby; you will know when the phone is transmitting and it most certainly isn't often.
So if you want to write an SMS, and send it now. a radio connection must be established. In GSM, this requires an SDCCH (Standalone dedicated control channel). This is a finite network resource (even if you are using it for 5 seconds or so, it is still a finite resource). In most cells, a static reservation of 8 SDCCHes exists only. Also, setting up this SDCCH involves other temporary channels - it occupies capacity on the AGCH (access grant channel) and RACH (random access channel - to establish the request in the first place). If it is an incoming SMS, it additionally requires capacity on the PCH (paging channel). All of these latter channels have particularly finite resources.
In UMTS ("3G"), the scenario is similar. SMS is typically delivered over the FACH (forward access channel) mapped to the S-CCPCH. The S-CCPCH has very limited capacity in most networks, and is being shared between other requests to establish channels, mobility updates from phones moving about, etc etc etc.
The point is SMS does consume finite network resources, and they are more finite than you think. Your assumption/myth might be valid if you can piggy back SMS onto the back of the (typical) hourly location updates that occur, but who wants their SMSes to all be buffered once an hour?