There are plenty of reasons to use all sorts of sizes of reeds - that's why there are all sorts of sizes of reeds.
The reed has to work in combination with three variables:
* mouthpiece tip opening - we use a harder reed with a close tip than with a wide tip. The reason is that a close tip doesn't require the reed to move much whereas an open tip has a long way for a reed to vibrate.
* ligature tightness - the tighter the ligature, the harder it is for the reed to vibrate therefor requiring a softer reed to get it going. A loose ligature is easy to get a reed to vibrate and therefor can take a harder reed.
* your embouchure - the stronger your mouth muscles, the easier it is for you to set a heavy reed into vibration. Weak mouth muscles need a soft reed.
We have to consider all three of those variables in order to find the ideal reed. Since most student level instruments come with a medium-open mouthpiece, and most students have a medium embouchure (assuming they ever practice), and most stock ligatures are neither loose nor tight -- most students use a medium reed (2.5 to 3) for an ideal sound.
But, if you change any of those variables very much then you need something different. I play on a very wide Pomarico crystal mouthpiece for Dixieland - that demands a soft reed (1.5) for the sound I need even though I practice several hours every day. Put that same reed on a regular student mouthpiece and it squeaks really loud.
If you play so strongly that you split a usual 3 then what it means is that you need to back off some or get a mouthpiece with a wider tip so it has room to move. It could also mean that your mouthpiece has a closer tip than a stock student and you really do need a harder reed. The widest Vandoren mouthpiece needs a 1 - 1.5 and the closest needs a 5 for ideal vibration.
Now, why the differences in mouthpieces? Tone and timbre. A hard reed sounds woody and warm. A soft reed sounds bright and jazzy. A hard reed answers slow though and a soft one very fast (like Dixieland). You get the mouthpiece that allows you to use a reed that produces a sound you want for your style and the music. The typical medium-open student mouthpiece and a 2.5-3 reed produces a good all-around compromise sound that works for pop, classical, and march music ok. Not great on any of them. Since you'd like to make a district wind ensemble, consider a nice Vandoren 5RV with a Vandoren 3-4 reed and you'll have a deep warm "traditional" clarinet sound that wind bands and orchestras like. Expect it to wear you out pretty easy though.
If you want a combination that gives you ability to last for a long time (endurance), a wide range, and a so-so compromise tone then look at the B45 with a 2.5 - 3 reed.
For a jazz/dixieland sound (Benny Goodman, Artie Shaw) and some fast answering with not much air, then look at the 5JB with a 1.5 reed (or a Pomarico wide tip).
There's no reason you can't own all of them (and others) for different combinations in different uses too. It's all a matter of getting your mouthpiece/reed combination ideal for the sound you want at the moment. Holst Suites? Nice, tight 5RV and a hard reed. In the Mood? Super wide and super soft.
The myth that "better players use harder reeds" is simply a myth. If it wasn't then Vandoren wouldn't make professional reeds at 1.5 to 5.0
PS: if you're using Rico (or any other inexpensive) reeds - it's time to switch to something decent like the Vandoren traditionals. [note: Vandoren reeds are harder than the same sized Rico]