When looking at equipment, you will see that they have a modifier, ranging from S being best, A being second, down to E being worst scaling, next to none. This scaling converts a percentage of that specific stat when you use that weapon or armor.
To see how much this bonus is actually affecting you, equip the item, then in the equipment screen, simply hover over it and look at the stats to the right. Say a weapon had 100 physical damage, with an A Strength rating and B Dexterity rating, with you having 40 of each. The weapon's damage will still be 100 physical damage, but beside that number will be a blue +x, the x being the bonus granted by the scaling.
While leveling your stats might appear miniscule when you look only at the stats card, you will notice a vast improvement if you were to wield a weapon with B scaling over a weapon with E scaling. Even if the weapon with E scaling appeared more powerful, in the end, the B scale would have the greatest potential.
In this regard, leveling Strength and Dexterity will provide the character with increased damage, but less versatility. It is not unheard of for a character to focus on multiple skills, such as being competent with melee and magic, or a magic user that has strong defensive capabilities.
A character who has split talents will not be as proficient in all of them, but will be better suited to take on many situations. As an example, the boss
Ichorous Earth, of Iron Keep, just before the Primal Bonfire, is a large boss that is present only in lava.
A melee character can, feasibly, combat this boss, but a magic or ranged user will have an easier time with it. A character that is proficient in many roles may have an even stronger upperhand. In truth, it is all a matter of preference and playstyle.
One thing you need to know about infusing with elemental attacks in DS2 is that it scales off of another ability score. With lightning, you'll be scaling off of your Faith. So, if your Faith is low, you won't be adding that much more damage.
http://darksouls2.wiki.fextralife.com/Upgrades#x%E2%80%8D%E2%80%8DWeapons%20Upgrade%20Chart:%E2%80%8D%E2%80%8D
Head there and read up on what different infusions will add for you. Just putting lightning or fire on something now won't just make it better. You'll need to be investing in other stats to make the infusion better for you. If your only big stats are Strength and Dexterity, then you might be better off just upgrading your weapon to +10 or get a better weapon that scales better.
Best Answer
In Dark Souls 2 you can consider infusions and the +N upgrades as two separate properties of the weapon. For example, if you have a +10 Claymore and infuse it with lightning, you'll have a +10 Lightning Claymore. Further, you can "uninfuse" it and it'll go back to just a +10 Claymore. There's no upgrading of the infusions themselves. That said, if you infuse a less than +10 weapon, you can still continue upgrading it to +10 with titanite, and it will retain its infusion as you upgrade it.
Whether or not to infuse a weapon is another question. Each infusion does something different to the weapon. Something like Lightning will take away some of the base damage on the weapon and replace it with lightning damage. This also affects the scaling. A purely strength based character probably wouldn't want to infuse with lightning, since doing so would likely lower the scaling off of strength and add scaling for lightning (which is affected by Faith). A Strength/Faith hybrid build might do a lightning infusion, though.
There's so many combinations, and each type of infusion works differently (Raw increases base damage but lowers scaling, Mundane adds scaling based on your lowest stat, etc). My best advice would be to experiment with different types, as there's not necessarily a "right" answer (keep in mind that you can preview the changes before you actually go ahead with the infusion). It depends a ton on your build and playstyle.