I have one and two MB150S combos, (recently scored the second one for super low $349 at Guitar Center West Hollywood on a new closeout with a $100 coupon).
I have the extension cab, second owner, bought several years ago to pair with the older MB150S.
I like the little cab for its light weight, portability and sturdy aluminum construction. It adds a little to the combo, but not enough that I take it out much.
Where I have enjoyed it is just as you describe: as an easy to live with practice and casual gig cab for switching heads.
I would rate the sound as mid-toned and mid fi, but it's the ease that makes it a nice piece to have around. Lightweight and portable is an understatement.
I was using it with an Ampeg portabass 250 head and it was a nice portable practice rig, a clever two-hand load in (gig back on back, cables in gig bag, GK in left hand and Ampeg in right).
I also used it as a jazz guitar cab, with the Ampeg. Tight jazz guitar voicing, clean.
Overall the cab is a good second item and very useful, flexible. Tone is average.