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Indeed, it is a stochastic way to simulate mobility evaluation. In the presence of other terms it should of course not be made so large that it dominates the total evaluation. Like explicit mobility terms should not dominate the evaluation. But its weight should not be set to zero either: properly weighted mobility might add more than 100 Elo to an engine.

Joker has no explicit mobility in its evaluation, and relies entirely on
this probabilistic mechanism to simulate it. The disadvantage is that,
because of the probabilistic nature, it is not 100% guaranteed to always
take the best decision. On rare occasions the single acceptable end leave
does draw a higher random bonus than one-hundred slightly better positions
in another branch. OTOH it is extremely cheap to implement, while explicit
mobility is very expensive. As a result, I might gain an extra ply in
search depth. And then it becomes superior to explicit mobility, as it
only counts tactically sound moves, rather than just every move. So it is
like safe mobility verified by a full Quiescence Search.

In my assesment, the probabilistic mobility adds more strength to Joker
than changing the Rook value by 50cP would add or subtract. This can be
easily verified by play-testing. It is possible to switch this evaluation
term off. In fact, you have to switch it on, but WinBoard does this by
default. To prevent it from being switched on, one should run WinBoard
with the command-line option /firstInitString='new'. (The default
setting is 'new\nrandom'. If Joker is running as second engine, you
will of course have to use /secondInitString='new'.)