You may find it easier and safer to deploy musketeers on the flanks of your army
where they can easily be moved to outflank your opponent. Musketeers are fire by rank so I find it better to not deploy in the front of your force at least not en masse because once you have to run them through your front line to reposition, you are losing time and you are affecting not only the musket fire but the formation of the unit/s it is running through.

Also, if you deploy musketeer or other long range missile units en masse and away from the rest of your units, you are able to draw many units away as the attacker becomes split between attacking a INF line or w/e you have and chasing your missile units about.

Since musketeers arent front line material at all, it makes more sense to use what strengths they DO have which is range and accuracy to your advantage rather than tempt the attacker to engage a crowd of mixed units at your front line. If you want to mix guns with swords use handgunners in your front line since they are not fire by rank (mixed results as always with handgunners)