Donald, the arranger keyboards are instruments which come with the ability to play pre-programmed accompaniment patterns in the key defined by a chord played on the keyboard with, say, left hand. For example, you can specify that you want to play Bossa-Nova style of accompaniment, press D-F-A keys, and, viola, the pre-programmed 8-piece band begins playing BossaNova, drums and all, in D-minor. What is nice, is that your right hand is free to play the melody of the sond you want, or improvise to your heart's content. And if you need to switch the accompaniment chord, you simply press the keys for the new chord, say A-C#-E-G, and the accompanying "band" switches to Amaj7 chord. This is very different from a regular sequencer, where you have to program the song into, and then play it back in the same key, or transposing the entire song, like a DJ would. Though, most arranger keyboards also come with a regular sequencer, which you can use to play midi files while taking a break from playing yourself.
The accompaniment styles (also called patterns) are usually generic, rather than specific to a single song, so that you can use them for multiple songs - though you do need to play or sing the melody. Some of the features which distinguish high-end from lower arranger keyboards are the quality and number of styles, the ability to add new styles (which you can purchase or create yourself, though not easily), and quality of sounds. Sound generation is the one area where you will typically (though not always) find the arranger keyboards more limited than a dedicated synth/workstation. The arrangers usually come with a large number (from several hundred to over a thousand) canned (sampled) sounds, but with only rudimentary abilities to modify them. Yes, you can layer several sounds as a rule, but not change individual ones to the same degree that you could, say on XP-80. I don't see it as a problem, since the arranger keyboards are designed as performance instruments, and the last thing you want to do during the performance is to start tweaking sounds.
The arranger keyboards are usually very strong in reproducing the acoustic sounds (pianos, string, grass, woodwinds) with little emphasis on the true synth sounds (like square wave, jet takeoff, etc) - these are usually relegated to the category of sound effects.
I do not compose much, though I like to improvise a lot. My first keyboard (other than a piano) was Korg Poly61 back in 1984. Since than time, I have gone through a number of keyboards from various manufactures, only to find that I really could not make musical use of all of their abilities of sound manipulation, of adjusting VCOs, VCFs, and the like. What I really wanted was some good-quality sounds of regular instruments, which is what the arranger keyboard has given me. It has also given me the ability to come up with a musical idea and in seconds have an 8-piece band accompany me while playing it. It is also a lot more fun to take to a party and entertain friends (or a paying audience).
I don't think that an arranger keyboard is for everyone, and your musical needs are undoubtedly different from mine. I would recommend that you find a music store which can demonstrate to you one or a few of the arranger keyboards, if the words above sound interesting to you. Then you can make up your own mind as to whether you can make use of the unique features of an arranger keyboard or if you are better off statying with a synthesizer workstation.