We have it, it's tested, but it's not documented in stap.1 nor stap -h.
After looking at the source a bit, I don't believe the '-M' option does anything or is tested. There is a 'merge' field in the translator's session object. Specifying '-M' does change its value, but no translator code appears to look at the value (except to make sure it isn't set with other incompatible options). It isn't really tested either. There is some basic command line option testing to make sure you can't specify '-M' with other options. It is also used when doing testing of the cache feature to make sure it changes the cache hash. But, I don't see a real functional test of '-M'. Perhaps since it doesn't really do anything, that's why it isn't documented. We should think about making it do something (assuming we can remember what it was supposed to do) or remove it altogether.
It looks like translator support for merge (STP_RELAYFS_MERGE) was removed in commit e65b03c170887fa9076119afd123a7384c12c630 back on 2007-03-14: * translate.cxx (emit_module_exit): Print warning using _stp_printf so it shows up at the end of all output. (translate_pass): Replace STP_RELAYFS with STP_BULKMODE. Eliminate STP_RELAYFS_MERGE. I'll remove all references to '-M'.
Fixed in commit 635035c.