This is a stylistic choice. If you feel that you are a competent keyboard player, then it is easier to simply perform parts on a keyboard and record them onto Reason's sequencer.

I should mention, I've never felt that this is Reason's strong point.

The more common way to use a keyboard controller with Reason is to record the MIDI from the keyboard controller onto a robust, primary sequencing/arranging application (think Nuendo, Cubase) and then Re-Wire into Reason, using it primarily as a slave. Then you can record all of your audio and MIDI in one place, on your primary DAW, where it can be modified.

Yes, you also have other options with the keyboard, such as hundreds of presets that can be used to easily control many of the knobs and oscillators on the reason instruments, and this can be very helpful.

If you want to get a cheap keyboard just to see, get one of these M-Audio Keystation 49 USB MIDI controllers. They are around $100, and I have basically sold everything else and just use this and a mouse for home use.

Now let's discuss the second option - scrapping the keyboard and doing everything with a mouse. As for inserting notes manually, I feel that I am an expert at it, but I've been doing it for about 15 years and also play many instruments. It is especially important to have a very strong knowledge of drums and theory if you want to attempt to manually plug in each note.

So to get back to the question, I don't think it is easier to make a song in Reason with simply a mouse unless you are making very straight-forward electronic music, or have a lot of knowledge about production, arranging, and musical computation. I think for more expessive music, it is even more difficult to create without a keyboard. But if you want COMPLETE control over every note, from dynamics, to length, to pitch, to modulation... everything... it not impossible to put every note in from scratch. But it isn't easy.

Here is a song I made that was created entirely with a mouse, to prove that you can still do great things without a keyboard with Reason 4:
http://shanebarber.com/on-it.mp3 (NSFW lyrics, MSI remix)

To hear something that was done primarily with a keyboard controller, you can hear/see this song I did for Project Runway:
http://www.youtube.com/watch?v=qMs297LGSHE

Let me know if you have more questions.

Having used Reason, Logic and Logic Express I can safely say that no matter what you are doing with music apps you need a MIDI keyboard. I am a big fan of M-Audio's Axiom 25 because it gives you a few keys to use while also providing pads and other things that you can hook up to various controllers in Reason. I find the pads useful for creating drum beats, though in reason, Redrum (a virtual software instrument) is really all you need. For anything other than drums with Reason I'd definitely get a MIDI keyboard. I've linked to the Axiom in the sources list, It is great because the keys do provide feedback and it is unique in terms of quality at it's price point.