Directly from SLF4J homepage "In fact, the binding between SLF4J and a given logging API implementation is performed statically at compile time of each binding."
Sure, it does not mean that you need to rempile your application - but you need to replace slf4j implementation jar with different one. Not a big tragedy, but anyway somewhat less flexible then completely dynamic binding.
Any may notice I do not try to say which of 2 (SLF4J or JCL) is better and should be used. I just say that both have their pro and cons which should be weighed in each case individually. Especially as it is not very complicated migrate back and force, see http://www.slf4j.org/legacy.html#jcl-over-slf4j