Here is the misconception: Active-active adds some more performance, so for them it is seen as a good thing.
LS in Multicast mode does add some throughput in sterile conditions without any significant load, but it also introduces some other limitations and caveats, such as SDF, Flash&ACK Sync requirements, loss of SecureXL acceleration, etc.
Unicast mode with two members means 70% of traffic being forwarded to the second member, so once more, increased performance here is questionable.
There is actually a document describing the limitations of LS mode: ClusterXL Load Sharing mode limitations and important notes
In addition, you can always refer to ClusterXL ATRG to the relevant chapters.
As for the official recommendations, you may have mentioned Check Point is very cautious about calling supported features as "non recommended" in any general sense. Each functionality can be required and productive in specific scenarios. For example, using unicast LS mode might help to cope with outdated switches and routers that cannot handle multicast traffic and gratuitous ARPs.
In my personal experience, however, the usual justification of using LS mode specifically in your region is usually "why the second box does not pass any traffic if we paid for it". It is not a proper engineering argument, in my book.
