Tiers are an extremely useful mechanism for RPG. It is an easy way to show the level of proficiency and master of a character. Tiers are not very different from school grades. You learn basic things in grade 9 and in grade 12, it is more advanced than grade 9 but some basics of grad 9 still exists.
Similarly, fireball is more advanced than flame blast yet the basics of flame blast is part of fireball. Logically speaking, you need to be able to cast continuous streams of fire from your hands before forming it into a projectile and aiming it at something else. The same goes for mana clash and mana drain. Mana clash involves mana alteration to cause damage and mana drain is the most basic form of mana alteration.
If you cannot imagine that, think about something like maths. You cannot learn how to do differential equations without learning multiplication, addition, indices and geometry. By the time you have mastered how to differentiate and integrate complex functions, things like 1+1, 3 x 2^4, tangents, etc seem dull, easy and somewhat pointless when without learning them, you cannot have mastered differential equations.
That is how mastery and learning of anything and everything works. Only naive people expect shortcuts. This is especially true of magic where mages learn magic and they get better at it over time. Naturally, they will have tiers for their spells to make the learning process easier.
Having said this, there tier systems are not perfect. Like Magebane mentioned, it makes no sense for a mage to improve earth spells to improve their lightning ones. It however, makes sense for a mage to improve on their basic lightning spells to boost and advance their lightning casting skills. This does not mean we should just do away entirely with tiers.
Additionally, it should be perfectly fine to have two paralysis spells that have different effects. Once again, using differential equations, you can solve a differential for a function using just geometry or just algebra or a combination of both. So similarly, if a mage wishes to paralyze a target, it makes sense that some would want to directly cast a spirit spell, some want to lay a creation glyph and some might want to petrify them.
Again, this is just how things work. This is how RPGs are supposed to work. You have to have different methods of doing things, you have to go through some form of learning curve. Not having either of these aspects is a form of streamlining. It hurts replayability and it hurts roleplaying.