Why choose floor standing speakers over bookshelf?

Lower, yes, louder, not necessarily. For most HT applications separate bookshelf mains and subs are the better choice, while floorstanders tend to have adequate lows for music only without subs. Probably an upgrade, for the price I can't imagine the drivers in the BIC are very good quality.

Depending on the floor-standing speakers you can also gain a greater depth of sound stage when listening to 2-channel audio. Not saying that there aren't great bookshelves out there that can obtain this same effect (B&W CM5, 805D, Sonus Faber Venere 1.5, etc.) but it's typical of a floor-standing speaker. I also agree with the above poster in regard to floor-standing speakers being able to reach lower frequencies than bookshelves so you will notice them being able to more accurately reproduce the low-end sounds.

All-in-all I believe that switching your setup would actually be an upgrade, but do consider what you'll be doing more in that room (more HT or more music) when choosing your floor-standing. Nothing wrong with Polk but I think it depends on their application.

In tower speakers, more drivers do mean less thermal compression, more surface area, and less distortion, so most of the time it does mean louder. However, if the tower has no additional drivers over the bookshelf speaker, there won't be an output advantage, but this is all assuming everything else is equal.

The main difference between towers loaded with a fifteen inch woofers, as is the case with the OP, and separate mains/subs with the subs loaded with fifteens, is the number of boxes. That in and of itself gives neither an advantage. What does give the separates an advantage is the ability to place the subs where they work best, which for HT is seldom, if ever, below the mains. In music only applications the lowest frequencies produced tend not to be too adversely affected by room modes, nor is wall or corner loading as beneficial as with LFE sourcing, so placement of the woofers below the midbasses and tweeters isn't as detrimental as it is in true HT applications.
Where the midbasses are concerned having more than one does potentially allow for increased output and lower distortion, but there's no need to get a floor stander to have more than one midbass.

If you can afford it get really nice towers and cross over to a really nice sub at 50hz. Next best is bookshelves with sub. Last place is Towers by themselves unless they're super high end and can do the lowest frequencies without a separate sub like those $5000 definitive technology speakers (forgot the name).

You don't need separate stands/mounts, so 100% of your money invested is going towards the speaker

I've been having this same debate for months. I feel like only the very upper end of bookshelves can really fill out the music like towers. I've also found that integrating a sub also isn't always easy to do well. If I had unlimited funds I would probably go with high end bookshelves, but in the end I have a feeling I'm going to end up with towers, no sub. If It appears to be more cost efficient to me. I mainly listen to 2 channel music.

1. i think towers just look better
2. most of the time you need speaker stands with bookshelves, dont need that with floorstanders
3. instead of space used by a speaker stand, why not add a couple more woofers to move more air?

thats how i look at it, oh, and did i mention towers just look better? lol

I have to admit I originally went with bookshelfs to keep my speakers out of harms way because I had kids but now that is no longer a concern since they've grown. I really like the flexiblity the powered sub provides and I like the fact that the room is not dominated by large speakers. The sound is better than any floorstanding that I have ever owned so no complaints there. So consider me a bookshelf/sub convert.

Any cabinet with identical multiple drivers gets 6dB additional output capability for each doubling of the driver count.