I can't be bothered to change spells, honestly. Heck, I even smack around rhinos with Forest Rage because it does more damage at T4 to them then the T1 that I didn't want to go collect resources for.
My fault? Of course it is.
Reality? Yeah. I can get a LOT of distance out of a few key spells. The lack of variety is due to both being expensive in my time and mission selections (particularly mid-tier when you're hunting down NPC buildings and windshelters for T2/3). As an example, I've been trying to hunt down Earth Essence, a mission only reward, for a while now for Forest Rage T5. I at best find 2 of these in any mission, usually 1, and then I don't have a lot of choice as there's only 1, maybe 2, missions with it.
Doing that for 4+ spells to 'keep my options open'? Bah, no. There's no reason for it. The spells, in general, are much of a sameness. The utility spells are quite useful, but that's an entirely different discussion. Combat spells vary very little. I don't use touch spells, getting in melee distance is built to be painful and possibly suicidal if you're wall-snagged. Whip spells really are more of use as a chainsaw then a combat ability due to cost, and that's on a very heavily -mp cost character. Instead of costing LESS mp due to distance, they cost MORE MP because of the damage capability. Nah, I'll hang back here and shoot and dodge, thanks. Less damage per strike, more Damage/time.
So, that leaves long range spells as your real 'need to upgrade'. You typically need *1*. Yep, one. Oh, there's a few things out there that do 99% resistance so have a backup plan somewhere, but really it could be anything that you got via tripping on the supplies while getting stuff for other things. I lean towards the high ROF items because they're the least likely to be dodged but even the slow high damage heavy thud ones are equivalent, really, when you come down to it. It's not like you're likely to use a slow heavy item for Urban Crawlers and a high-speed one for anti-bat, which would at least add a little variety... because the slow heavy doesn't increase your DPS... why worry about it?
Add to that the need to have a platform assistant on your second mouse key 90% of the time. Oh, sure if you've got one of them newfangled 5 button mouses or something you're good to go. I don't, and clicking the middle button always is a challenge in 'Don't SPIN IT!!!... crap. Get back there before this bat kills me!'
So, typically you've only got one combat spell loaded and ready, the rest are utilities.
Why was I going to go play with other spells again? Do they really do anything different? There's a few out there that are interesting, though, I agree, and goofing off with them at Tier 1 was fun. Do they give me enough versatility to really go out of my way bringing those spells to T5 and having to hunt down the specialty items? Heck, most of the time folks on Nomads seem to be hunting down Burnt Embers for their PRIMARY spell... really, we're going to do this for a situational spell that I've got to pause to go collect?
*shrugs* Until combat spells are truly different in results, feel, and methodology, and add significant tactical value to different situations, I don't think you'll ever see much experimentation because the investment time can be too high.