-having panniers negates the use of a backpack.
-you take your pannier off when you lock up your bike and carry it inside - because, well, it has all your stuff in it. (assuming you lock up outside or can't leave it in a room/office safely)
-grocery panniers and commuting panniers are two different things. one is designed to carry shopping bags, the other misc stuff like clothes
-you don't have to buy giant panniers. clothes fold up to be remarkably compact when done right.
i'm a teacher and i have this trunk bag/pannier combo:
sorry for the picture size - thats amazon's fault.
i can fit a folder of papers upright in the panniers (about perfect height) plus a bunch of other stuff because the pockets are wide too. in the trunk i put shoes, phone, keys, etc. honestly, it's way more space than i ned on a daily basis because all my clothes for the day fit in one pannier. and many times, depending on the clothes and if i didn't need to take shoes, i could roll up everything and fit it all in the trunk part, and didn't even have to use the panniers and could keep them rolled up and put away (assuming i wasn't bringing any papers to and from). that's basically how it was every day when i was commuting to teach summer school.
-the topeak MTX system just lets me unclip and slide the whole thing out at once, and carry it with me. it also has a shoulder strap. it can be awkward a bit if you have the panniers down, but its not a big deal, really.
i wouldn't put a laptop in it though.