Panesar may be poor, and don't get me wrong I am not a Panesar fan in any way, but Flower obviously backs him to be his second choice, and I am a big Flower fan, so he must be doing something right!
Panesar is what can loosely be termed a horses for courses selection - Swann is very much England's number one spinner and will remain so pretty much as long as he is able to get his arm around. Meanwhile, whatever you make think of Panesar, he has a good level of international experience and a reasonably calm head on his (bowling) shoulders - so in the circumstances where he is likely to play you either have:
1. a solid spinner on a raging turner who, crucially, turns it the opposite way to Swann and at a very different pace and tajectory or...
2. a guy who can reasonably be expected to block up an end as the team workhorse.
Now, where Swann not on teh scene, you'd need to consider other variables, such as their ability to take wickets on intermediate pitches, their wider contribution etc, in which case you may well choose someone other than Monty...